Ergonomics aside, what I particularly like is the "mature" sensor (shared with the D4, IIRC). Its IQ is definitely better than predecessors' such as D700 and D800, and not significantly inferior to successors'.
When I first tried the excellent Z7, I moved my trusted Summicron-R 50/2 from the Df to the Z7+FTZ. The results were sure excellent, but not to the extent they would make the Df obsolete. Some day I'll enjoy stabilization and focus assist, but for the time being, if my Df broke down, I'd probably buy another one.