1. Don't leave your wrist straps on and then walk around with them dangling off your arms like you're some kind of "athlete." Nobody gives a shit, dude, and you can't possibly be using them for every exercise.
2. Don't stand on shit if you don't need to. I can't imagine a single reason for a guy to stand on top of a box squat box to do shrugs unless he's 3'7" and can't reach the hooks where the bar is (in which case he should just f-ing deadlift it). Yet I see this every day.
3. If there are ten different spots to do a particular exercise, and only one guy using one area, don't use the area right next to him. Learn some f-ing spacial awareness.
4. Put your shit away. You're not strong enough for anyone to have to clean up after you. I've been in gyms for the better part of 25+ years. I'm not taking plates off the bar for a sixteen year old kid.
5. Shut the f--k up.
6. There is no reason on God's green earth for the settings on a glute-ham raise, every day, to be switched to having the footpads at the lowest possible setting and the half moon pad extended to where like 8 holes are showing. Last time I looked, I didn't see any 8'2" guys at my gym, and there is no exercise I can envision that requires a GHR to be set this way -- so I assume it's being used by people who have no idea what it is.
7. Stop posturing. There are 20 people in the gym who can whip your ass in a fight.
8. If you're standing in line for something and it's crowded and cramped, don't repeatedly turn around. It irritates the guy standing behind you like you would not believe. This has nothing to do with the gym, but it's sound advice.
9. While I'm at it, if you're walking in a crowded pedestrian through-way, don't stop walking. If you need to do this, pull over to the side.
10. When you take a leak, wash your f-ing hands, especially if there's someone else in the bathroom who will know you didn't do so.
That's just from this morning.