The difference to me between a top-level pro and a seasoned am is not so much in ability as much as confidence and consistent execution. Basically it boils down to a solid mental game as MJ said, the ability to perform at a high level under pressure.
But sometimes the difference between pros and ams isn't all that huge. I played in my first PDGA sanctioned event this past Spring, the St. Louis Open. After two lukewarm rounds the first day, including one on my home course, I shot a hot round(the best round of my life) on Sunday shooting a 64(par 66) on a very long and challenging course. That easily took 1st for the round in INT and tied the top ADV score, putting me in the final 9 and taking 3rd overall. I found out later that Nikko had lost his cool and posted a 60 on the same course. I was just in the zone that day, all my shots were on and I didn't hardly miss a putt. The difference between me and the pros is that they can perform on that level day in and day out, when I can only do it once or twice a season.