To be honest, yeah the sweat thing annoys me
But the main thing that annoys me is when people bitch about other people not exercising "properly." as in, not running fast enough, walking on the treadmill, spending the whole time on the bike, etc... okay it might not be good enough for your standards but at least they're doing something and if they're doing it wrong it's only them who won't reap the benefits.
In short, it annoys me when people don't mind their own.
Of course, if someone's not using equipment properly and you tell them how to use it properly, that's a different matter.