People that don't clean the machines after they're done. There's a towel and sanitizer right next to them, they sweat all over them, but they won't clean them!!! Yuck! Buncha slobs.
People that sit at a weight machine/bench for 15 minutes between lifts daydreaming. They'll do like 10 reps, stop, then just sit there staring straight ahead. WTH???
People that put their mouth on the water fountian...that is wrong on so many levels.
Posers- people that set the weights on the machines a few notches higher before they get off of them to make it look like they were lifting more than they actually were.
Fakers- People that take the trouble to get dressed, go to the gym, get their sports drink, magazine, and MP3 player, then use the cardio machines for exactly 3 minutes, do maybe one or two lifts on a weight machine, then either stop to watch tv or leave. What was the point of that???? I see people do this every time I'm at the gym.
People that make fun of fat/super skinny people that are trying to make themselves better. If I see a 400 guy on a tredmill, I think "Good for him! He's taking control of himself!" If I see a 70 pound nerdy guy, I think "Great, he's trying to improve himself to make himself feel better." Many other people start snickering or poking fun at them- uncool.
Finally, my biggest peave- People that TALK while they're lifting. I don't mean chatting with friends about the game or whatever, I mean people that say stuff like "COME ON BABY! OH YEAH!! GGHHHAAARRRRRRR I'M HUGE!!!!!!" I don't know if I should start laughing or just give them a simple "Will you STFU you tard!!!!"
Yes, I'm pretty annoyed when I work out

I've been doing it three times a week for the last 5 months, so I've seen my share of idiots.