They have natural talent, but they also work a lot more. I have played with guys with enough talent to play on tour. I have played with guys who have been exempted for a few years on tour. Hard work is a big part of it. I shot 32 on the front nine this past Saturday. I lost it on the back because I made one bad swing and couldn't recover. Thats what practice does, one swing doesn't kill the round, when I am practicing like I need (like) to, I can recover from bad swings, if not, I can't. Thats what makes these guys so good, they can recover from bad swings and bad shots better than the normal 1 or 2 handicap.
They also have 'it' between the ears. I once played with a guy who shot 64 goofing around with his son one afternoon. He never hit a bad shot and ran putts in from everywhere. We were playing the back tees, about 7200 yards. I watched him play at TPC Wakefield for the Rex Open Nationwide event, and he shot 79-75. He just doesn't have the mental toughness to play at that level.
The two keys: Mental toughness and hard work. Talent is certainly big, but I think a good number of people who worked as hard as Tiger or many other guys out there could be successful on some professional tour (not necessarily the PGA, but Euro, or Japan or somewhere).