Still interested in seeing what anime/manga drives people insane with disappointment so lemme hear it! XD
You can also compare the manga versus the anime version. Which one did you like better? Did any of them stick to the original story or just run off on it's own?

I have some. First things first as everyone said Inuyasha. And I've got a BIG rant for this one. At first it was a neat and simple anime, agitated modern girl and near villainous feudal dog boy set out on an adventure to reclaim shards that grant power. They battle all sorts of monsters and dog boy gets a powerful sword. It was good. They later meet a perverted monk, a whiney fox brat, and a demon huntress who join with them, the group a little too large for my liking but they were cool enough (except for fox brat). Eventually Naraku shows up and we learn he is the cause of everyone's problems. They fight his minions and even Naraku himself yet Naraku finds a way to escape/or is too powerful. dog boy then goes for a number of filler episodes to find a way to upgrade his sword, then more filler for upgrading said sword, finally powerful enough to take out Naraku yet never gives the death blow. Naraku would SLOWLY escape and the other characters would just stand there with their thumbs up their butts and watch and not move until he is nearly or completely out of site. Rinse and repeat for the WHOLE series. If dog boy's sword needs to be upgraded that many times, why not just get another sword except make sure that this sword would be giga powerful and when Naraku tries to escape have dog boy pull his thumbs out of his butt and swing the death blow and be done with Naraku. For dog boy and Kagome's love troubles take them to modern Japan and lock them in a room in a love hotel and play Isaac Hays music. Anyway the anime ending had me saying every swear word under the sun and even the manga ending was dragged out too long. For these crimes Inuyasha is now to be called simply dog boy and these shall never captilazed even at the beginning of a sentence!
The next one is the ending of Hellsing. I only saw the first two episodes of the anime and got bored so I'll be ranting about the ending of the manga.
SPOILERS!!! DO NOT READ AHEAD IF YOU.... heck with it! Read ahead! You're better off making up your own ending and I'm sparing you the pain I felt when I read the end of the manga!
First of all Hell had broken loose in Londen. Vampire Nazis were slaughtering everyone they came across, major landmarks were being destroyed and it looked bleak. Then out of the dark ocean emerged a vessal. It came closer and controling that ship was nonother than our favorite antihero: Alucard. His sadistic grin and and wicked eyes showed he was ready to take them on. As the ship gets closer I could hear Yngvie Malmsteen's songs Evil Eye and Far Beyond The Sun were playing in my head. I squeeled with glee as he lands and wipes out all of those who oppose him. This drags on a bit but the ending has to be kick ass! Anderson shows up and battles Alucard, even mutating himself into a half tree monster and yet Alucard triumphs! You can't wait to see what happens next! Next is where everything falls face first into skunk crap. In the traditional case of "the butler did it" Walter suddenly turns traitor, claims to be brainwashed and tries to kill Alucard. He fails and it was a dying Nazi who poisoned the blood that Alucard drank to become superpowerful that almost did Alucard in. Walter is eventually defeated and when Integra battled the head Nazi, he turned out to be a robot! A ROBOT! Was Hirano Kohta watching Alien when he thought of that? What was he thinking!?! I know there is some scifi in Hellsing but not THAT much scifi! Then Alucard disappears for a number of years and then finally returns to an elderly Integra and it ends. Reading the ending made my blood turn cold. It was the biggest WTF since dog boy! Anyway I'm going to go blow off some steam from bringing back such bad memories of manga/anime ruined. I'll be back later to tell more that made me POed. Sorry if this was long but these things make me want to rip my clothes off and run out into a cornfield while howling at the moon. Let me know what you all think.

For anime, I was disappointed with Tsubasa Chronicle, Avenger, and Casshern Sins.

For manga, I don't think I've ever been disappointed with anything more than D.Gray-Man.
I LOVED the manga and then the author hit a break point and came back with a horrible chapter.
I cried a little on the inside.

Originally Posted by Adderly

SPOILERS

For Fruits Basket! It started off so good and the ending was just like, "WTF?" for the anime that is. I mean, that's not even touching the real plot of the manga. How can that be an anime series? I heartily enforce another season or something coming out to finish what the manga started.

And then the manga! I loooved Akito. He was so awesome, and then to find out that he is really a girl. It's insanely craptastic. I did like, however, that Kyo and Tohru get together. I mean, Yuki is okay and all but everyone liked the rat and nobody liked the cat. :] But still. Severely disappointed in the ending of the anime and that Akito was a girl.

i know what you mean about the anime. the earlier episodes were awesome, but the ending just wasn't enough!!! it ended to suddenly. and i read somewhere that Akito was a girl, altho i haven't read that far yet. i find that odd that it turned out like that.

one manga that i was disappointed with the ending was Yurara. the manga was great, and if there isn't an anime yet, there should be. but there wasn't enough at the ending. it, too was too sudden.

Alrighty! Now for some more! Hirano made a prequel to Hellsing except for some reason he made Alucard look like a Japanese school girl. I kid you not.
Anyway, another disappointment was Haruhi Suzumiya. My complaint about this is not so much the show itself but more of the fan(atic)s that overhype it as the greatest thing since PS2. The show itself is okay but not the greatest anime ever.
Evangelion for the most part was an excellent show yet the ending...(puts finger into mouth). I know it's about Shinji overcoming his insecurities but what about the other characters? The end seems to present Shinji as the only important character and the others are ignored. And what about the Evas? And Nerv? They are forgotten. The manga luckily develops differently from the anime and so do the movies. At least they showed what happened after the Angels were defeated.
Sensual Phrase was interesting for awhile but the story would just repeat itself (similiar to dog boy unfortunately). Someone tries to get between Aine and Sakuya. Aine becomes confused or afraid that said someone would ruin Sakuya's band. Sakuya knows something is wrong yet Aine is too much of a crybaby to tell him what the problem is. Eventually like a magician pulling a rabbit out of a hat, Sakuya would find a way to defeat said person, often Aine has to step in and stop Sakuya from outright killing said person.

The last season of Rurouni Kenshin after the arc where he fights that religious guy...the episodes were quite disappointing and boring. I loved the rest of the series though <3

Another disappointment was the ending to the School Rumble manga. Thruought the manga series I enjoyed all the hilarious hijinks and agnst of the characters; Harima trying to get Tenma's affection even though she loves someone else, Yakumo and Sawachika both having a crush on Harima, and somewhere near the ned of the manga it was revealed silent militarist girl (can't think of name right now) developed a crush on the guy with glasses (can't think of his name either) who was class president and has a crush on Yakumo and a slight crush on Suo. The last part never got played out. Tenma travels to America to be with the guy she loves. He has memory problems and this is never resolved. Harima, instead of developing any feelings for the other two girls that like him, continues to have a crush on Tenma and becomes a dead beat bum. The ending was an extreme disappointment; it felt rushed, slopped together and with many plots unresolved just made an otherwise excellent manga into a sad and tangled ending. Reading the end ruined my day. School Rumble Z was essentially worthless. Hardly anything in story and even the plots in it contradicted each other. If you read the manga just skip the last few chapters and don't bother with Z at all. You are better off making up your own ending.

Third season of Rurouni Kenshin, as stated before by TheYaoiButterfly was a HUUUGE disappointment to me. The whole Christians fighting against the establishment bit in the beginning was.... okay by me for the most part. It was when they got into the killer feng shui that they lost me. SO AWFUL. I'm glad they stopped when they did. Uggggghh.

Also, the Chrno Crusade anime was SOOOOOO disappointing. I love the manga. Love it love it love it. But... UGH! Okay, there will be some SPOILARS NAOW! First of all, the anime RUINED Aion's character. I loved him in the manga. He was a sympathetic villain, one who wanted to do something good for his family but took all the wrong roads in order to do so. I loved him. But they completely changed him in the anime. Suddenly he's a malicious, unfriendly, slightly perverse, run-of-the-mill bad guy. He had no saving graces. He was terrible. Then there was the ending of the show itself. Rosette would NEVER give up like she did in the end of the anime. She's such a strong female character that having her just up and disappear with Chrno while she waits to die was so bad. Then there's the fact that it was just an all around depressing ending. Aion didn't die, Joshua never fully regained his right mind, Father Remington lost his faith, and Chrno and Rosette died. Oh, gracious. The manga's ending was SO much better. It was bittersweet rather then being totally soul-crushing. *siiiiiigh*

Fruits Basket anime was completely ruined. Manga is way better. How can you just leave so much out when you have so much in the manga?
Tsubasa was okay. I have only seen the anime. It was too bad that they didn't finish it. I would have loved to see how it ended. I am not really in to manga that go pass 20 volumes, which is probably why I haven't read the Inuyasha or Naruto manga.
I honestly loved the 5th season of Sailor Moon manga than anime. More there and better story.

Another one I just thought of is Elfen Lied. It starts out well enough with the psychotic psychic Lucy killing everyone but then she becomes a silly girl with no memories and reduced to an infantile status, saying only nyu and running around naked. Then along comes Kohta who, as the series progresses (or regresses) becomes more and more of an unlikable jerk. With him is his incestuous cousin and they find the infantile Lucy who they called Nyu simply because they can't think of a better name for her. They take her to a hotel they are looking after (thruought the series the hotel is never opened thus making it convenient for a harem anime/manga the series eventually becomes) and the first thing Nyu does is pees on the floor. Later on Kohta has to change Nyu's clothes and gets caught by his cousin in an akward moment (and these akward moments happen thruought the whole series, from beginning to near the end). Eventually they are joined by a cute girl with a dog and later girl who wears diapers. Nyu develops and obsession with breasts thus leading to more unnecassary fan service.
The villains are stupid rock heads. They are for the most part dumber than Jessie and James from Team Rocket. They know how and where to find Lucy yet don't really do anything till near the end of the series. Bando is an unlikable jerk and near the end he dies a hero's death only this is ruined by the fact he is brought back to life. The main villain is a retard who wants to be a god of the declonius yet he tortures them thruought the series and when he is killed he dies the death of more of a side character rather than a villain. There are more complaints I have about this series and I'll mention them later.

Samurai Champloo's manga sucks. I'm pretty sure Samurai Champloo was a anime first but the manga just sucked, and it was only two volumes. XD The anime is WAY better and I felt that the manga left some stuff out...

Well I still love Trigun but I really wish they had given it a better ending. Shesh Vash wins the battle and comes back with his brother over his shoulder but nothing else is settled. You can win the battle but still lose the war. Knives isn't just going to accept defeat. He's too stubborn and psychotic. They really needed to follow through and settle the story.

OMG YES! T.T it sucks cuz i LOVE anime/manga, so i hate when i go to watch a new series and im all excited and then it sucks. >.<

I adore InuYasha! i was totally sucked into it for the majority of the last 2 years, finding every episode online, watching them all, reading all the books, i was completely addicted! and then i FINALLY found the last episode on an anime site, and it ended and i even said out loud "...what?" i was so confused at first. i remember going back to the page where all the episodes of InuYasha were listed and refreshing the page over and over, as if that would add more episodes, thinking "NO, that cant be it! theres gotta be more!" *refresh refresh refresh*

InuYasha. Great anime, crappy ending.

The ending to Inuyasha was disappointing.....at least they're continuing it and Inuyasha The Final Act is not wasting time on fillers and drawn out battles.

They're announcing a new Code Geass project. I would be incredibly disappointed if it was R3 or some continuation from R2. That is just saying to me, "people loved R2, let's milk it for all its worth and use the vague ending from R2 to bring Lelouch back to life.".

Lelouch is dead, get over it. He is not the dude driving the wagon!

I loved Code Geass but if they're somehow bringing Lelouch back to life with some continuation, it's instantly going on my hate list. They would have to work really hard to get out since I would still watch it....HOPING it isn't a piece of crap.

But then again, the new project they announced was probably the manga "Renya of the Dark" so whatever.

Ok warning BIG spoiler for Trigun if you haven't seen the show yet i suggest you dont read the rest of this go right now and watch it...all...marathon it if you have to it's AMAZING!!!!

ok as for the most disappointing moment...I HATED that Wolfwood died!!! ok ok i understand how it was perfect for the story and that it was almost necissary but come on!!! he was without a doubt my favorite character. And the fact that he left poor Miley is just depressing. While i'm on the topic of shows that kill off the best characters...Gurren Lagann...Greatest man EVER!!!! KAMINA!!!!! why kill him!? again i know it's perfect for the story but I guess I always connect with the people who die...hope that doesn't mean anything XD

Anyways thanks for giving me a place to rant about that ^_^

I've probably had more, but this is one that I'll always think of,
in the manga Q.ko-chan,
there are 2 volumes; I like the manga,
but it's ending was cut because apparently,
since there wasn't any dialouge for the translator to translate in the last pages,
they cut the manga to the last spoken sentece DDDD:

I want to get the Japanese version of the second volume because of this,
just so I can see the ending >__>

im a tottal otaku so i have a few.
Inuyasha, Anime
Crono Crusade, Manga
Friuts Basket, Anime
Wolfs Rain, Anime, Manga
Trinidy Blood, Anime

I was extremely disappointed with how Escaflowne ended. All signs pointed to Hitomi and Van hooking up at the end, but they didn't! Instead, all she did was return home. I was beyond pissed. I had also thought that Ash and Misty would've kissed or something at the end of the second Pokémon movie. When they didn't, I was like "What the hell?! Why not?!" I suppose it's my own fault for expecting every anime to have a lot of romance in it.

whoo Ranma is on top haha;) I didn't like the ending!!!

I do not like the habit japanese shows have with being inconclusive. Like for a few episodes in the end it gets serious, almost seems like something is about to change and then WHAM! It's like it was episode 1 all over again.
So I'll just list some anime that disappointed me from the very start instead :/

DNAngel: What the deuce? What happened to the characters and plot from the manga is beyond me. I don't like when anime is 100% faithful to the manga but they could atleast have made it INTERESTING!

Chobits: The original manga had heart, and I love the questions about why people strive for humanoid robots and what is important in a relashionship between humans and 'things' alike. The anime was just, so bad I can't even begin to describe. They made a WHOLE episode about Chii looking for panties.

Samurai Girl - Real Bout Highschool: Just a sad excuse for Keiji Gotoh to draw panties I guess. Taking all the characters from the manga (though that itself is based on novels) and throwing in some sad attempt at a fantasy/sci-fi plot device. Most of the time I was trying to understand what was going on, and it was so horribly bland.

And hugest disappointment for me ever? TOKYOPOP LISCENSING SHOWS/MANGA I LOVE.
Stop destroying everything, please.

----------

Originally Posted by SuperPotpurri

I've probably had more, but this is one that I'll always think of,
in the manga Q.ko-chan,
there are 2 volumes; I like the manga,
but it's ending was cut because apparently,
since there wasn't any dialouge for the translator to translate in the last pages,
they cut the manga to the last spoken sentece DDDD:

WHAT THE? That's just wrong! So the pages werent good enough for the book out of the sheer reason that there wasnt any dialog? Last time I checked images can speak for themselves. I was thinking of picking up Q.ko-chan but now I don't feel like it XD (perhaps it's availible in german?)

InuYasha for being ridiculously anticlimactic. Paradise Kiss anime for being a "coming of age story" rather than a "Whirlwind Love story", it's all in like the second half of the last episode.

And I wanted Ouron High School Host Club to go on forever, but like all good series, it had to come to an end sooner rather than later.