Those who ignore the 30 minute time limit for cardio machines (and/or cover the time up with their towel). Last time I was waiting for a machine, there was someone who had been on the machine well over 30 minutes (she ended up stopping after an hour).

I'm never sure whether to say something. I usually end up just waiting and someone else will give it up. But - when there are multiple people doing the long workout thing, it can be a 20+ minute wait. I get that people need to workout longer than 30 minutes - when I do, I get off, wait for another machine, get back on. Not sure what the correct etiquette is.