The main features of flying in first class domestically are wider,
(much) more comfortable seats, more elbow room, more personal attention
from the flight attendants, beer/wine/liquor available at no extra
charge, better food service - and these are true of most any major
carrier which offers first class. Also, first class pax board first
and often are served a pre-departure beverage.

Frequent fliers using upgrade coupons are the main occupants of first
class these days and most appreciate the extra room and comforts.

The longer the route, the more first class is appreciated and
(generally) the better the service. Long haul domestic flights
(JFK/LAX or ORD/HNL, for example) can rival international service in
first class and, of course, as was mentioned by others intercontinental
flights are where you see a real difference in first class service.