For me it's all about the space. I'm 6'3" and about 255lbs, a disturbing amount of which is no longer muscle!

I hate flying coach as a result. The seats are narrow, the arm rests hard, and the legroom is truly scary for someone my size. My shoulders are so broad that they overflow the center seat in those rare occasions I get stuck there on standby by about 3 inches on either side. I t's not just me that's miserable, it's my seatmates.
Free booze, a quieter ride (farther away from the engines), and the occasional snack don't hurt.
B--