I treat myself to F once or twice a year.
For me, it is (in this order):
The seat (ample space, storage room, comfort of the seat itself, leg room galore for stretching)
The better bed (bigger, longer, wider, sturdier (not cobbled together with a foot rest), good bedding and PJs)
The privacy (hardly see or interact with other pax the whole flight, no drop-down screens or needing to climb over others)
The smaller cabin (usually less noise, less traffic to or waiting at the toilets, etc.)
The access to comfortable lounges on the ground
The larger baggage allowance
The service (food, drinks, snacks, faster and more individual service with less pax, etc.)