The ability to use GPUs, whether to upgrade to GF or to BF.
The seat is actually pretty good. The layout, especially on 777s and 747s is awful, especially if you're stuck in the middle.
The service, whether on the ground or in the air, is nothing short of appalling. In any class. The IFL is an exception if you're lucky enough to gain access.
I have only paid for C/J on UA when the only other choice was Y on UA and the flight was over 7 hours.