Two things really irk me.
I don't mind playing with high-handicap golfers. Honestly, I don't hang out with a bunch of aspiring pros--I like to play with friends who enjoy the game, and most of them aren't threatening to break 90. But guys who stand over a shot for 2 minutes, and take six practice swings, only to hit it 15 yards, and then repeat the routine. HIT THE FREAKING BALL ALREADY!! If you want to work on plane, or setting the club, show up 45 mins early and grab a bucket.
People who aren't paying attention to whose turn it is. Be ready to hit when it's your turn to hit. (Frequently it's the same people described above.) After I've hit my approach, and I'm thinking about the birdie putt I'm about to hit, it totally throws off the groove to look over and see you, sitting in your cart, looking at me looking at you, and then you suddenly realize your ball isn't going to magically levitate and fly to the hole. Maybe you were hoping I'd come over there and hit it for you? Nope. So now you get out of the cart, walk to the ball and look at the lie and shot. Then you walk back to the cart, grab a stick. Put it back, grab another. Walk to the ball and address. Back up and take a practice swing. And another. And another. Walk back to the cart and grab the first club you picked up. Back to the ball for four or five more practice swings. Then you duff the shot about 20 yards. Back to the cart, clean the club, put it in the bag. Drive 20 yards. Sit there in your cart, looking at me looking at you, and wondering who is out.