Also, I cant stand the people who leave their weights everywhere!! Put your crap away when you are done. I dont know how many times Ive had to go look for something or spend 10 minutes cleaning up after someone before I can work out. Its not that hard to buy weights back on the rack!!!!!
That's a huge pet peeve of mine as well, and falls under my general catch-all category of lack of consideration for others. Seriously, if you can't take a few seconds to break down/re-rack your equipment, then you shouldn't be using it in the first place. I'm also not a fan of people who slam (read: actively slam, not just let fall to the floor) dumbbells down between sets, mostly for the risk you then run of damaging the equipment. If you can't handle the weight well enough to prevent it from crashing to the floor and potentially breaking in half (which, btw, I've seen happen), you probably need to go lighter.
Oh, and yeah, seconded on the "curls in the squat rack" point. Although let me qualify that statement: if there are four or five racks and all of them are open, you can use one for whatever the hell you want. But the minute the others fill up, and particularly if you see people waiting on a rack, take yourself over to the EZ curl bars and have at it.