TWEENS. My dear God I fucking hate them; there needs to be a separate set of activities in a seperate area to occupy them, especially at night when all the over 18 panels are going on and they can't get into the rave because there's too many people. My major pet peeves have to do with panels and the people in them. I hate people talking loudly and just being obnoxious and stupid in panels. I'm okay with quiet talking/whispering, particularly if it relates to the discussion. Going to a panel just to have a place to sit and talk is not cool, particularly when the talking is so loud nobody can hear what the panelists have to say. I also hate it when people go to a panel just because they're bored and have nothing to do. Vying with the funk for #1 grossest thing is people not dressing according to their body types.
People who are impatient, you know; we're all in the same boat together so we may as well TRY to have fun while we're waiting around :).
This reminds me of the con disaster of Anime-Expo '07 AMVs which we waited in line for at least 3 hours I would say. We ended up making friends with all sorts of people making silly signs and telling jokes, pulled out the DS'es and started up Mario Kart matches, etc. People just need to realize that not everything is going to go as planned and you need to go with the flow and enjoy yourself ^_^