I think the one I tend to abuse is sweat and standing too close to the dumbbell rack.
The sweat thing I am generally ontop of when it comes to machines and cardio equipment, but not so much benches. I am just used to my overcrowded school gym, by the time I walk away to get the cleaner and paper towel, it's already taken by someone else. I don't bring towels with me, one because I generally bring nothing to the gym, and two I'd just forget anyway.
The dumbbell thing I try to be good about too, but sometimes when I am drop setting curls it tend to get involved in getting it done quickly that I forget to step away.
I think my biggest peeve in the gym are people who make excessive noise, whether it's excessive grunting (the older crowd is often at fault for this), slamming weights (definitely the younger show offs), or just talking nonsense for too long.
That and people who just stand around and brag all day. Can't stand that shit, no one cares what weight you're using, a pat on the back is cool, but don't fucking chat up how great you are for half an hour.
Oh and one more, people who get pissy if you ask them (politely mind you) for either a spot or to use a piece of equipment.The other day I asked a guy to spot me on bench press, waited till he was between sets, he was the only bigger guy in the gym, so I figured it'd be alright.
He barely spotted me, he spent the entire time looking away, headphones on, didn't help on the last rep. Meh.