I have been trying to get 100% game completion in GTA IV recently. I am just at 71% right now.
The most difficult thing about getting 100% is committing time to the game. I get sidetracked so easily while playing it. I think the worst thing, though, is trying to win a game of pool. I. Just. Can't. Seem. To. Win.
I get close every time, but I always end up losing. FML