Hot guys with really fugly wives and girlfriends.
People tying up the stretch mats with planks and ab work.
People doing single pulley exercises surrounded by single pulley exercise machines on the only dual pulley machine.
People supersetting on three or more machines which are never near each other.
Trainers teaching their clients bad gym etiquette, like not re-racking, collecting nests of the most popular fixed barbells and dumbbells and leaving them in the most far flung, counterintuitive corners of the gym, and not letting others work in during their long chat breaks. Speaking of which...
People's inability to let you work in no matter how nicely you ask, specifically:
People with ipods:
If you try and ask them (why else would you interrupt them fiddling with their playlists on their five minute long rests between sets?) they make a big show of dramatically peeling away an earbud and looking extra annoyed before telling you they have two more sets - if they even bother addressing you.
Men training with women:
One of them's always the alpha. If their break between sets is less than two minutes it's the guy, who'll act as if you're pissing on his leg and go all mano a mano to show off to the girl. If their break is longer than two minutes it's the girl, who'll deign to acknowledge your existence by shooting you a dirty look for disrespecting their relationship by daring to interrupt their conversation before ignoring you to resume her yapping.