Go play Armok I. I'm sure you'll find the experience enlightening. This is what the game was before Toady decided to scrap the graphics.
/snarky
IMHO, we don't need 'just' 3D. We need 3D tailored to the game in a way that it enhances the experience and gives us more power over the game.
Now, I'm not saying even simple 3D wouldn't help. You can, after all, do the same kind of z-level view, maybe with a visible grid to help the player see what actually is on their current z-level. This would give us the same control scheme we already have in the game (good control over a single z-level) and ability to see more than one z-level (solving the problem of finding those sneaky goblins on a hill). Good enough for the overworld and possibly some simple constructions.
Now, let's take a look where this would have problems. Say you wanted to build a pyramid within a pyramid. You'd never be able to see the whole inner pyramid, because when you look from above, you see the outer one and looking at the lower z-levels gives you only a cross-section. My idea here is to introduce some kind of view-depth-peeling mechanism, where z-levels are abstracted away when looking at complex structures and you rather switch between peel-layers. It would need a lot of development, testing and usability work to get it right, but in the end, it could solve the worst usability problems DF currently has (or at least the problems I see as worst).
Also, creature models, item models or any of that is fluff. Simple sprites with letter textures rendered on them would be just about right as a start.