What would you consider the biggest nuisance at an anime convention? No offense to anyone responsible for these things.
Regarding the second choice: Over the past four years, I've been attending voice actor panels on a regular basis and at more than half of them someone asked how to get into voice acting and the panelist/panelists had to spend 20 minutes giving all the details on how to do it. I've never found anything wrong with people having this goal and I think it's cool how VAs have inspired so many people to want to get involved, but when you hear that question at almost every panel and have to keep hearing the same kind of answer again and again, at least a third of the audience will be annoyed by it because it's been asked at a million panels and most of the audience wants to hear as many different questions as they can, which allows at least a few less people to ask their own questions, which is especially frustrating if they have something really good to say. I've met one voice actor (the awesome Michael McConnohie) who makes fliers he puts on each chair in the panel room giving his own insight on it, which I thought was real smart and I'm starting to recommend other VAs to do the same thing.
Regarding the third choice:
I will admit that I think it's repetitive seeing so many people cosplaying as characters from Fullmetal Alchemist, Bleach, Naruto, Final Fantasy 7 and Ouran, but as common as it is, I have no complaints about it because in my opinion, it's best for cosplayers to do characters they like and are most comfortable dressing as because that way, they're being themselves more than they would if they dressed as someone from a lesser-known anime. Although if you're going to cosplay, that should be your purpose, not because the character is popular (no offense to those who do that). However, it is nice when people dress as different characters.