Aside from being able to attach the best lenses available for my type of photography (obviously):

-Bright viewfinder with 100% coverage
-MLU !!!
-Focusing screen(s) that makes focusing easier
-Time/Bulb settings
-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.