Guys who are lifting dumbbells that are like 10-12k, and then just let them drop to the floor after every set with an audible smash that startles others.
People who put dumbbells on the rack where ever they feel like, rather than in order of weight or even in corresponding pairs.
People who leave entire puddles of sweat on benches/equipment/seats and never, EVER wipe up after themselves.
Gyms that play Nicki Minaj songs...