I mean when you can paint over a prominent or deep parts of a surface and adjust the brush penetration on the fly smoothly. With more pressure, bigger size, a certain velocity. Like a real brush in the real world.
What we have right now is an ability to paint over every pixel above the middle (zero) height or over anything deeper/ bellow zero with a kind of abrupt treshhold on zero level. It works better in microvertex mode but in the per pixel mode the thing is pretty useless.
It's very important imo, a key thing for me actually. I like 3dcoat three channel painting and material feature but still use Zbrush mostly for texturing because I don't have this brush behavior here.
ps."Dry brushing" allows to do some dirty spots, worn/ glossy edges etc. And the more/less in cavity is not a decent replacement.