Someone who understands the huge gulf between the best club players and the pros on telly. He'll be competitive, but won't treat a bad shot as the end of the world and bring everyone else down with a temper tantrum or a sulk (although I must say that the sound of a well thrown club can sometimes be a thing of beauty..)
Someone who is sensitive to his playing partners' enjoyment of the round. He will praise your good shots, commiserate with your unlucky breaks, keep a line on your ball, and always remember that it is not all about him.
Someone with a sense of theatre who who is prepared to take on a few outrageous shots now and then, to call them in advance, and now and then to pull them off. Needless to say he will join in the hoots of derision when they don't come off!
Slow players! Never ready when it is his turn to play, always makes 3 practice swings, glove on/off all throughout the round and no awareness of how this affects his playing partners or the group behind. It is a particular kind of selfishness.
Someone who is obsessed with their own game, to the extent that the entirety of their conversation consists of immediate post mortems of the shot they've just played, how they played last week, what their last lesson was all about, how their new driver suits their swing speed etc etc. Don't care how good/bad they are, it's equally annoying.
Matchplay opponents who hole one long putt after another to beat you! This is true even if they are rather apologetic about their good fortune!