As a customer, I wouldadd the following points.
1. The "million dollar door" at T5. Google that one! (Or search the threads here). Only first class passengers can use that one.
2. Space. Personal space. On the plane and on the ground
3. With the space on the plane comes the opportunity to have a more personal feel about the relationship with the CC. They may or may not like you, and vice-versa, and that doesn't matter most of the time, but at least you feel that they are doing what they do for you on a personal basis.
4. Lounge access and quality. Generally (but not always) much better. e.g. BA at JFK.