For me? Personally? All of the minors that have no manners. I don't mind the ones that have them, but all the bloody green badges with absolutely no manners to speak of. They think that because they are under the age of 18, they entitled to do whatever they want. I had three photos ruined by greenbadges. I was up on the fish, late at night, and they all decided to try and climb and get up there too. While the person was trying to take the picture. And I asked them three times to get down, and they just asked me who I thought I was, and why I thought that I had the right to tell them what to do.
There went that photo op. I barely get pictures taken of me at con as is, even when I do have a really good cosplay.
Also, that's another thing that annoys me. I know that people are looking for certain cosplayers, but do you know how much it picks up someones day to be asked to have a picture taken of them? It's like someone shoved your own personal sunbeam into a persons heart. It's a beautiful feeling. Especially if they are wearing a nice outfit, but aren't a character from a series. People, stop to actually think that not just the 'popular' cosplays need pictures taken. Even if you don't KNOW a series, perhaps stopping to ask for their picture would actually really help their day out.
I ended up sitting in the park, on saturday night, just before all the minors were kicked out. I was sitting with a group of nearly twenty cosplayers, tucked away in the park, who had never had their picture taken the whole convention.
So i guess what annoys me most is that people think they are all that at conventions, and don't stop to consider other peoples feelings.