Some of the most annoying things I've seen in fanfics and roleplays have probably been said already, but are as follows:
1. Undeveloped story or underdeveloped story - Seriously, if you just slap together three crossovers of two of your favorite things plus the plot of the movie you just went to see, that's not a good story.
2. Undeveloped OC's - There's a point where 'overly shy, social outcast with a supreme talent that involves success every time' starts to get a little old, and it started right after the fandom began, so you missed the train. Static oc's also annoy me. The story is there to develop the character in some form or another, so if you write 10 chapters and your character doesn't change at all, you're either trying to say that your character is supposed to be static (and should probably be just a minor character), you don't know how to fix it, or you just don't want to.
3. XXX shippings - I think this one's been said to death and if it hasn't, we're all thinking it already. I'll leave its corpse where it rests.
4. Awful grammar/spelling - Nothing kills the mood and interest of a story quicker than 3rd grade writing. Well, apart from crappy and static characters, crappy story... List goes on, but this just makes me try and find something less productive to do for the rest of the day. I mean, I'll be honest... My grammar isn't awful, but it's not the best. You're not rushing to text your friends while you're driving, hoping some cop doesn't see you or you swerve over and hit a bus. At least try to make an effort to look like you're trying. That's far more tolerable than just trollfic'ing.
5. Trying to sneak in references to other things where they don't belong - You really shouldn't try to sneak it in there. It doesn't matter whether you really liked that quote from a movie or not, but trying to use main or memorable quotes from other things doesn't work out so well for your originality. So don't make your character stand in the middle of the bridge and yell "you shall not pass," or at least try and be obscure or strategic about it.
6. Unnatural crossovers - Mass Effect x How To Train Your Dragon makes just about as much sense as some of the other wonderful crossovers we've all had to endure. Stop it. Please.
7. Fallout - Just stop. Please. It's starting to get ridiculous and it's nowhere near the already-established canon that is completely free and available to read in-depth with author and storyboarder's notes online. If you're going to make a Fallout fic, follow the canon we already have. Stop twisting it into some creature that looks more like a gelatinous amoeba draped over a stale sack of flour.
8. School settings - Isn't something along the lines of roughly 10 to 20 years of school on average enough? Reading about it just makes me want to barf. I liked school, and I still do, but when you live through it it just seems so... unappealing. Yet it still manages to make some of the longest-lasting (and arguably lowest quality) roleplays and stories.