I'd love more support for tilesets. Like, make it possible to have a away to display levers as little lever-symbols without messing up ó and ò in text. I played a long time with mayday's set, but after a while it got very annoying to have random objects in the middle of text or tree-symbols in raised drawbridges. So basically, what's done for creatures in graphic sets (having graphics for say, a craftsdwarf, an axedwarf, a bowgoblin, etc. without messing up text or the tiles for other creatures), only for furniture, buildings and all other in-game stuff. Give us the possibility to make separate graphics for every game object, not only creatures, and the modders will do the rest. It would make it possible to have very detailed and advanced tilesets but doesn't mess up the traditional ASCII display.
Currently I play with a 12x12 ASCII set that I made myself. It's basically the standard set, only slightly enlarged, put into a square and with lots of pseudo-antialiasing. I'd probably keep using it, even if there were more possibilities, because I got used to it. But I imagine, better tilesets (which are not possible right now) would help new players to understand the game instead of screaming "Waaah, Matrix!" and turning off their computer right away.
3D graphics on the other hand... no, that's just not Dwarf Fortress and also out of the frame of what's possible for Toady. Even something rather simple (compared to completely rendered 3d graphics) like Stonesense's sprite based pseudo-3d would need a rewrite of way too many things to work. Opening up the interface for modification will probably be done in the future and make some improvements possible, but only Toady knows, what the far future brings for DF.
Generally, I like ASCII graphics... I could write several pages about the psychology of pictograms and visual abstraction, but others will explain (or already have) that much better. I just prefer it and that's not even, because I grew up with roguelikes (I didn't), but because it produces relatively little visual clutter. If I can have a simple 12x12 symbol or a detailed 16x16 graphic which mean the very same, I'd choose the simple symbol any time.
Just imagine you're standing in front of two toilet doors, male and female... there's probably a very simplified version of gender symbols on each door, which makes it possible to choose the right one. Now imagine the same two doors but on each one is a detailed drawing or photo of a man or woman... does that make understanding the meaning and deciding which door to open any easier? For me it wouldn't, especially after a few beers.
Now imagine the same with 256 doors (ok, unlikely in a toilet...), each having its own little symbol. I'd prefer something simple that can be understood at a glance instead of more complicated stuff. The same applies to DF tilesets. The simpler, the better, as long as there is a way to make a visual connection (╤ does look somewhat like a table and g obviously does not stand for "elephant")
Vattic's symbols actually do a good job of characterizing the entities without making things too cluttered.