Hogging the Equipment
It's the grown-up equivalent of toy-sharing, yet so many of us have a hard time wrapping our heads around it. Gym equipment isn't limitless, and at least some of the time other people are waiting their turn politely. Many gyms post signs gently reminding users of time limits on the high-traffic equipment, like treadmills or elliptical machines, but plenty of users gently ignore such reminders. If the place is a ghost town, by all means run for two hours like the human gazelle that you are. But if someone else is in the queue, mind your manners and let someone else have a chance.
It's also completely uncool to hide equipment until you're ready to use it. I went to a weight training class once and was dumbfounded to discover that all of the under-15 pound weights were MIA. A bit of sleuthing discovered them hidden behind a stack of yoga mats because another woman was "saving them for her friends." Incidentally, they never showed. Some friends! Loosen the reins a little bit and realize that, most likely, the equipment you need will be available to you. Even if it's not, it's hardly the end of the world.
My brother is a five-day-a-week gym-goer (showoff!), who lets nearly every irritation roll right off his back. Everything, that is, except for the guy who "slurps at the water fountain forever," as a line of thirsty exercisers builds up behind him. Next on Joe's list are the "bottle-fillers," who load up their enormous containers at the fountain, when everyone else in line probably only needs a few seconds for a hydration fix. If that's you, fill up at home or wait until the line is done.