Here are a couple things that I like:
Meters: Really partial to those nice on the nicer Teac/Tascam stuff.
Pitch Control: I really like pitch control so I don't have to retune to play along with the tape.
Memory Points: A couple memory points to quickly goto.
Transport: I like the nice and easy movement of the head assembly, although the clunk of a relay/solenoid has it's charm. Ha, maybe smooth transport with a dummy solenoid that doesn't do anything except add the charm.
Noise Reduction: HX-pro or something similar.
Speed: Usually only use the standard speed. However, if there is a second speed it should have enough pitch control to match all of the other two speed decks.
FF/Rew: Super fast fast forward and rewind times.
Front Panel: How about a front panel that looks similar to the Vintage receivers, cool lights, lighted switches, things that go bump in the night.... (that one is for Stuwee)
I am sure there are more....
Robert