1) Seeing the best players in the world play courses I would love to play. There is hardly a course in the Open Rota that I wouldn't go to great lengths to play. Outside the majors, there are maybe half a dozen PGA Tour stops that I would pay to play even if I were right across the street. And some of the PGA Championship venues are no better than a run-of-the-mill Tour stop.
2) Seeing more shotmaking per hour than you'll see per week in most US tournaments. The weather, the tight lies and most of all the firm conditions force the players to play funky-cool shots that would be low-percentage aberrations in a typical PGA Tour round.
3) I like seeing such a rich mix of PGA Tour stars, top European Tour players and internationals, not to mention the occasional remarkable performance by an amateur. It may not be considered the strongest field in golf but it's mighty strong and easily the most diverse.
4) The Open is very often played within sight of water, making for interesting scenery in the background. Plus the galleries are filled with a wider variety of spectators than the usual US Tour mix of corporate types and drunken boors (not that there aren't drunken boors at the Open as well).