Well one of the best things is a fully flexible ticket. This means if you don't turn up for your booked flight you just book another one.
Use of lounge without having to be a member of the airlines frequent flier club. Some facilities in the First class Lounge are pretty good too. The champagne bar in BA T5 is spectacular. Plus being able to 'fine dine' prior to flight. Also arrivals lounges are good.
Some airlines to some destinations provide a limo to and from airport. Separate check in facilities usually without a que.
The fact you are usually treated as a VIP rather than a bit of livestock is good too.
Extra baggage allowances, flat beds, better food and wine etc.