Fundamentally, it's legroom. And the longer the flight, the more it matters. Also, the need to be able to sit beside someone and still not have to pull my arms in and worry about armrests etc like we do in E.

Everything else is a bonus to me. Boarding early, priority bags, lounges etc are nice, but fundamentally it's the physical comfort I am after and it makes a HUGE difference to my trips on long haul flights because it means the difference between arriving refreshed and arriving as a total wreck.