Playing df for several years now.
Biggest problems for me are:
1) Boredom and lack of challenge. Once you learn everything, you got nothing to do. Game should have either some objective or make you really struggle for survive, with ever-increasing complexity as fortress grow and inevitable doom in the end. Sure you always can go with self-imposed challenges or mega-projects, but here comes 2nd problem:
2) Rapid slow-down with area/material/pop increase. I'm programmer myself and even tried to write some my own roguelikes, so i do understand why it is like that, but there's always a way to optimize.
a) With path-finding you can make optional caches for example. Its definitely preferrable for us, who have 4GB+ memory.
b) Modern pc's are multi-core and threading for such CPU-consuming application is a must. At least have interface running its own thread so it wouldn't lag so much.
Getting just 2x increase in fps would make game MUCH more playable.
3) Too much micromanagement and some unrealistic concepts.
I'm sure everyone knows the pain of skill management, esp in migrants-arrive time. Or building and furnituring rooms for these migrants. While there are external tools to make it easier, they are 'cheating' and do not run on linux.
Even worse is production management. Plants/free barrels/stockpile space/drinks for example. It would definitely help to have some simple in-game scripting for such, like scheduling some job when some kind of stock reaches some point, and cancelling it when it reaches another point.
Ex. "Plant plump helmets when theres less than 500 till theres 800", "Brew drinks when theres less than 500 till theres 800", etc.
Also, would like to point that i DID enjoy learning the game, and 'simplier' DF just would not be DF anymore.
Same goes for ASCII graphics. It does what it is intended to - displays the world to player in a non-obtrusive way and lets his imagination do the rest. Having _optional_ official tilesets could increase amount of players, but myself i would definitely prefer ASCII to be default, and not suffer any regressions for the tilesets.
Oh, and wtf happened with old accounts? I see my old posts marked as 'guest' for some reason.