I think I'm much more concerned about perspective. Of course, the best idea I think would be to have a game that holds true to its predecessors, but in 3D -- same gameplay, same perspective, etc. However, I'm willing to shrug off changes to TB, as I prefer RTwP anyway. So such a change wouldn't bother me, although I acknowledge it would annoy the hell out of some people.
But the change that would be a deal-breaker for me is perspective. I want isometric, or at the very least some fascimile, like what the NWN camera evolved into. I very much need to pull back and see the battlefield and the NPCs that I command. If the game only offers a first-person view, then no matter how well-done every other aspect of the game is, I cannot buy it.