So, by the time I'm done with this list, I'm pretty sure none of you will possibly be able to comprehend why I even come to anime conventions, but here goes. This is in a very particular order btw.
#1: People who assume I've watched a particular anime I'm not cosplaying from. I have watched a few...and enjoyed even fewer. I would loathe to have an encyclopedic knowledge of every single one...so why would I inherently know any particular one?
#2: Unfitting cosplays. If it isn't you, then don't do it, unless you can do it in a manner that's comical.
#3: When my friends tell me I should take a "break" from Gaming and check out the rest of the con. No thanks!
#4: Sloppiness in general. Whether it's a cosplay, a tournament, a live performance, or whatever. If it's sloppy, and the people responsible aren't even trying to fix it, that annoys me.
#5: Being able to find everyone except for the one person I'm actually looking for. I like to get to know people who have neat personalities, and cons are a great space to initiate a meeting between yourself and someone from the forums or wherever whom you find interesting without having to be really creepy or conspicuous. Those people, however, tend to be good at not bumping into me...while I tend to run into people like my friend David every time I leave gaming for more than 10 minutes, or I run into a friend from out of town whom I haven't seen in months when I wander over to the arcade at 1am.
#6: People who think they understand something that they clearly don't understand, and then getting frustrated when you try to inform them on the subject.
#7: Having to be "the man", and bring down the hammer on an issue, especially when it's a really nice/cool person. I always feel kind of bad when I have to tell someone they need to get off of a console and give someone else a turn, or when I have to tell someone, "sorry, I can't let you check out M-rated games because you're a minor," when they're, like, 16 or 17.
#8: Spending money. I HATE spending money. If someone had just raided my bank account and told me, "yeah, you're going to Sakuracon; I'll manage the expenses; just enjoy yourself" I would have been a lot happier there. I was really wound up because I spent a lot of money for, essentially, 20 or 30 games of DDR, 16 hours of work, and a dance.
There are other things that annoy me at cons, but they're more general. I'm a pretty irritable person I guess.