Things that used to fuck me off at the gym:

People talking to you when you are training
people getting in the way of an exercise - ive actually hit somone with a dumbell a few times to prove a point
people not being observant -- ie you are using a bench, get up to grab a drink from the water fountain and somone sits on your bench instead of asking if you are done with it
somone using a machine for a ridiculous amount of time
3 people training together and above
people on mobile phones when you want their bench