Aside from being able to attach the best lenses available for my type of photography (obviously):
-Bright viewfinder with 100% coverage
-Focusing screen(s) that makes focusing easier
-Standard mechanical cable release socket
-Intuitive, simple controls
Personally I've always found DOF preview next to useless. But it is always available on cameras with the above functionality anyway in case you need it.
100% coverage? That would narrow the field considerably....
Contax RTSiii and a handful of others, but they are all far too complicated for my liking.
Don't know of any vintage (pre 1980) slr's with 100% coverage.