I don't consider myself to be a gym rat and I may not know everything in the gym, but through the years I've definitely picked up on proper gym etiquette...and I get pretty frustrated when others don't follow them. Here are some basic rules:
1. Re-rack your weights
It's pretty self-explanatory. If you picked up a dumbbell from the rack..then put it back at the same location! I go to a newly opened LA fitness, and everything is clean, but one of the problems that the gym faced really quickly is that the dumbbells weren't where they belonged. I mean, there's a label of how many pounds each dumbbell weighs on the rack..it's really not hard to put it back is it? It seems like a stupid rule but it just takes unnecessary time and effort looking for the right weight. Not only does this apply to dumbbells, but plates too. If you loaded your barbell with plates..then put the damn plates back.
2. Don't hog the machine/bench/etc
Clearly if there are only 2-3 machines/benches in the gym, there's gonna be people waiting for it. I mean I don't mind waiting a minute or two, but I get annoyed if I see someone just sitting on a bench for up to 5 minutes, just using their phones or talking to people when there are clearly people waiting. I like to superset or triple set, so I make sure that others know that they can use my area when I'm not there. I don't just sit in one location. You shouldn't be resting for more than a minute anyways.
3. Clean up after yourselves
I'm assuming people are just unaware of how much they sweat at the gym, but if you've left a puddle of your sweat on a bench or the mat, it takes ~30 seconds to go grab some paper towels, sanitizer and clean up after yourselves.
I'm sure there's more, but I feel like following these three, easy steps will make the gym a more pleasant place for everyone!