I don't know if it might come off a little snobbish, but a REALLY big pet peeve of mine is the quality of a person's OC art.
I usually have realistic art when I RP and I'm fine with the whole anime thing (to an extent), but I positively loathe
to Rp with someone who's character image looks like it was drawn by a 3 year old. If your OC is a stick figure by design then
ok, but otherwise, I like when a little more effort is put into the art...

Another is when people get offended for their characters. Not sure if I worded that correctly... There was this one RP I was in
a few years ago where I was playing a villain in a wasteland. Came across one of the "heroes" and my character called them
mediocre or something along those lines. Point is the villain insulted the "hero." I then got a LOOOOOONG PM from an angry
writer as to why I would dare insult the integrity of their character. Their character was top of the line in blah blah blah and they
could kill me with a flick of the wrist and blah blah blah, they just droned on. I ended up killing off my own character because the
writer couldn't separate his/her own feelings from the character's and I didn't want to be a part of that.