I think for me the differential has to be between VTT and video game, I lived NWN 2 and would love a current gen version of it, however it's a video game and not what I'd want in a VTT.
I like to use a VTT to enhance the imagination and fill in some finer details, not the other way around where the imagination fills in the finer details of a VTT.
Having said all that I think there is definitely room for improvement on the current offerings and some great suggestions on this thread.
So for me:
A) ease of use, this means more modular maps and better ruleset integration
B) cross device support, make it run equally well on iOS, Android and Windows, even better if it's browser based
C) integrated lighting on objects - if I add a column to the map I'd love it to have all the lighting parameters built in, rather than me having to add them.
D) dynamic lighting and environmental effects- if I describe that a storm rolls in, id like to be able to reflect this on the VTT
E) scalability - mentioned previously, let me be able to zoom out for exploration mode and in for combat
F) open API - let the community be able to tweak it