I think my biggest pet peeve definitely stems from post quality. I'm not one to care about post size. You can write how much or how little you want so long as I have something to work with.

Sometimes I find that I'll make a post which in a couple paragraphs or whatever will describe the scenario, what my character is doing, etc. And then I'll get a response (typically it's a one-liner, but not always) where the other might write something like

John watches Alex.

Or something really simple like that, which gives me no context to respond to. Did this character just enter the room? What's he feeling? How long was he watching for? I have no idea. And that drives me insane.

I tend to find that this behaviour is often linked to persons who leave it up to me to carry the weight of the story, which is fine sometimes. Everybody has days when they can't come up with great ideas, but for the entire story not even including one teensy tiny little thought? I tend to find that kills an RP for me. Because then when I hit the proverbial wall, we're left sitting there RPing, but not really doing anything.

PWEEP also touched on something for me that I find bothersome. In group rps, it's not as big of a deal for me because there's typically a lot going on and every now and then somebody gets confused. It's pretty easy to set straight.

However, I'm a frequent 1x1 RPer and I find that a lot of times what ends up happening is I will make a post and if my partner doesn't like what I've written, they'll just completely ignore what I've written and write something else in place of it because they seem to think that's the better route to go, rather than just take a minute and talk to me OOC about it.