Showing up less than 15 minutes before your tee time
Working at a golf course for years, I know this drives the starter and those in the shop nuts. It potentially puts the rest of the tee sheet in danger of delay. The 15-minute period is actually generous; most courses would prefer golfers be on the course 30 minutes before balls are in the air.
Putting with too many balls on the practice green
If you’re by yourself, feel free to go nuts. If the area is crowded, no more than two practice balls, or else you're hogging real estate.
Cart-path only? Carry more than one club to your shot
Having to run back to find the right stick will slow things up.
Looking longer than three minutes for a lost ball
We get it: Golf balls are expensive, and it's nice you're risking the threat of poison ivy to find another's ball. But there's a difference between making an honest effort and belaboring the point. After three minutes, come to peace with the ball's disappearance and move on.
Talking to someone's ball
Well intentioned as it is, it's annoying, and to some, disingenuous.
Standing behind someone as they putt
Even if not directly behind them, overtly going to school on someone's putt is untoward.
Walking in a player’s “through line”
Many amateurs are unaware that a putter’s line extends two-three feet past the hole. This is essential, as if a putt misses long, the through line is where the ball will end up, and it is the path for a golfer to finish the hole out. Next time you watch a tour event, keep an eye on Phil Mickelson. He'll routinely practice a putt on the through line side of the cup.
Placing bag on a tee box
Congrats, you decided to walk over ride. Alas, many that shoulder their sticks fail to keep their stand off the tee box. There's the potential to scuff up the hitting area, but of greater note, bags can be distracting if it's in a player's striking vision. Speaking of bags...
Be mindful of where you place your bag
For those walking, before you put your bag down and walk to the green, take a couple extra seconds to put your bag in the most logical place for you to pick it up and move onto the next hole. This can save a lot of time after you're done holing out -- so the group behind you isn't waiting longer than they need to be.
Lack of divot pattern on the driving range
There are differing opinions on the right method -- some belief your divots should be in a row, others in vertical lines -- but there's unquestionably a wrong one, as scattering your shots chews up more turf than you need.
If you're terribly behind on a match play hole, pick up
The beauty of match play is that strokes don't carry over to the next hole. If its apparent you've already lost, pocket the balata and get moving.
Checking your phone too much
It's one thing to check a score or send a text between holes. But continually attached to your phone goes against the spirit of being with friends or one with nature. Even if not causing an audible disruption, you're telling your group that you find them tedious and boring.