1. The extreme grunter. Low-level grunts are one thing. Just don't be the idiot who grunts as loud as they can on every rep. You're not impressing anyone. You're just annoying everyone.
2. The equipment hog. Don't bring half the barbells over to your spot all at once. And don't try to claim more than one machine at a time. If you want a private gym, put one in your house. Otherwise, be mindful that other people need to use stuff too.
3. The creep who stares at other people. Guys are the main offenders. Yes, women look good in yoga pants, but it doesn't mean you can gawk. Just do what everyone else does . . . take a quick look, and avert your eyes.
4. The clueless parent. This one's a little less common. But if you bring your kids to the gym, don't let them run wild and hang off the equipment. If they're old enough to be there, work out WITH them. Letting them do their own thing annoys everyone, and it's dangerous.
5. The narcissist. Meaning the person who can't stop staring in the mirror. If you're doing it during an exercise to work on your form, that's fine. But if you're just checking yourself out to see how SWOLE you are, you look like an idiot.