It's all about short game and consistency. tons of guys have loads of talent--just look at any division college player. But how many of these guys end up on the pro tour? Only the ones who are consistent enough to make it, and often time this isn't a top-ranked college player. Where do guys like Jerry Kelly and Jay Williamson come from? Lots of practice and an ability to repeat good shots. Every decent golfer hits plenty of good shots. Its the ones who can hit good shots (not even great shots) with the most consistency and who can make the most out of them with a good short game that make the money.
To do these two simple things seems to require a tremendous amount of confidence and mental focus to eliminate any ideas of hitting bad shots. No matter how much talent you have, you can always talk yourself into a bad shot. Only mental toughness can overcome this.
A side note: Labeling Charles Howell III as an 'average' putter is all relative. The worst putter on the pro tour makes more putts than 99% of all golfers. Just notice if a pro is 5 for 10 from 8 to 20 feet in a round, it is regarding as a bad putting day, whereas if an amateur makes five putts outside of 8 feet, he is 'making everything' and 'having the putting round of his life.' Putting is often derided as lucky in amateur circles, while it is the ultimate test of skill at the pro level.