I think a lot of it is the way the course presents such a fine line between glory and disaster.
The back 9 in particular might be the ultimate set of risk/reward holes in golf. You have to hit great shots on 10 and 11 just to make pars, and getting bold on either hole can mean birdie or double. Then, the stretch from 13-16 is just wonderful.
13 hasn't really reared its head this week yet, but it likely will tomorrow. Someone will make an eagle and jump into contention. Someone will hook a tee shot into the trees or a second into the hazard and drop out.
14 is fantastic. The way a perfect shot can stick close to the pin, while one that misses by a foot or two feeds into 3-putt territory makes it one of the rare holes in the world on which angles still matter for elite players.
15 is perhaps the most delicate "power" second shot in the game. There's so much disaster lurking on that hole, even for players who lay up. But it's also a real eagle opportunity. Like 13, it creates lots of birdies and lots of bogeys or worse.
16 has those brilliant green contours that can feed a smart shot toward its target and repel poorly struck or thought out shots away from it.
Then, 17 and 18 are just tough holes that force a champion to make great swings to get it home.
Augusta National must have the best final 9 holes in tournament golf. The potential for huge swings is all over the place, and the demands of the player are about as complete a test of his game as those posed by any other course in the world. Everything from driving to long irons to short irons to chipping to putting is tested sternly in a stretch of 9 holes. That's what I think makes the tournament so consistently exciting. Unlike a US Open, you can go low. But disaster also lurks unlike at any other tournament I can think of.