In order of irritation:
1) Not re-racking
2) Trying to hog more than one machine
3) Clicking the dumbells together on dumbell presses
4) Leaving sweat on the bench
5) Terrible form
6) Spending too much time on the squat rack
I actually like it when I catch someone in the act of doing one of these things, because I won' t hesitate to tell them about it. I think some people just don't know any better so do me a favor, if you see it, correct it!
I usually don't correct people on their form, most people would think you're a jerk, but it hurts to see it.
I avoid most of these by going during off-peak times when I can