Fast and easy belt changes, fast and easy tool rest adjustment, those are the two big ones for me. Ill elaborate a bit on both.
Changing a belt should take less than 10 seconds, not counting the time it takes to get the belt to behave, and should require no tools of any sorts. Essentially, all you should have to do is change the belts is press down to release the tension on the belt, slide one off and the other on, with no wrenches or screw####### or whatever. Tension on the belt should also be constant even as the belt stretches, avoiding the use of something like a threaded tensioner that remains stationary until adjusted. A gas spring pushing up on a hinged arm with the tracking wheel would be ideal, since gas springs provide a constant force throughout their range, and the length of the arm could be adjusted for leverage to make releasing the tension easier.
As far as the tool rest goes, i prefer something solid and flat, no fancy adjustable angles and whatnot. Flat spot for the work, attached at a right angle to the tooling arm. The important things are being able to adjust the rest in and out to/away from the platen or whatever easily, and again without tools. Ideally, a single handwheel/lever/whatever locks the tool rest in place, and can be loosened, the rest pulled in/out/replaced, and resecured in under 10 seconds, without the use of any tools.
In case you couldnt guess, im a huge fan of the KMG design, it checks off everything id want in a grinder, with the exception of the tool rest. Adjusting the tool rest i believe requires a wrench or screwdriver. My grinder is actually based on these plans:
http://dcknives.blogspot.com/p/2-x-72-belt-grinder.html
I modified the design a bit to have a hinged arm instead of a pillar for the tensioner, but the rest of the design is solid, in my opinion. The way the tooling arm and workrest arm are setup are genius in my decision, quick twist on a bolt release either for adjustment or replacement, like if you wanted to go from a platen to a contact wheel.
Give me a bit and ill try to get the CAD files for my grinder thrown up somewhere, might give you some ideas