Photographer pet peeve: When the photographer doesn't listen to any of your feedback. I'm still rather uncomfortable as it is getting my picture taken by strangers, it doesn't help if you're a camera nazi. When I politely mention I am uncomfortable with a certain pose or angle and don't want to do it, I really dislike a photographer trying to vehemently argue me into doing what I'm uncomfortable doing! On that note, two dozen pictures looking up my nose or having me in some weird pose are not going to look good. Don't try to convince me they will.
From the photographer's perspective, it's nice when people can offer variety on their own. It's awesome when you go to photograph someone, and they have an arsenal of things they want to try. You're not going to get amazing photos most of the time if the cosplayer is going to stand there, with the same blank expression, and trying to do the same pose, the entire time. Sometimes cosplayers should get reference and practice their character posing beforehand. Plus, it'll help you figure out poses that make you look good, and will show off the costume you worked so hard on.
Cosplayer pet peeve: I probably take more stupid, goofy pictures than serious ones. It's just more fun to do when you're in costume! But it just bugs me when I see a photoshoot of a serious male character, and through the entire thing the cosplayer is looking overly girly and SMILING. Not one picture that really resembles how the character would pose or look.
Seriously, that's the one thing that'll make me rant. Maybe it's my theater background, I don't know. But it just seems like you're missing the point if you haven't taken a single picture that contains the feel/persona of your character.