Hmm. I think that the UI needs a large number of little improvements and a few major ones before I'd say I liked it. I'd agree that it's not horrible, but a number of things annoy me about it. The following list is not exhaustive.
-It takes a long while to create megaprojects and any large constructions, since you can only build stuff only on top of other stuff after it gets built and you need to design it so that dwarves won't lock themselves in corners.
-Dwarves cornering themselves all too easily through digging/constructing, unless you make careful preparations each and every time you do so. Blinking notification of dwarves who lock themselves in 4x4 rooms or smaller could really help - though improving the AI so it didn't do it in the first place would be even better.
-It's impossible to do anything to or view anything from multiple dwarves at once. I understand Dwarf Therapist is a stopgap fan solution for that, though.
-The burrows system is rather hard to use, especially when you're carving out new areas for your fortress. Being able to designate burrows in unmined rock would help, as well as having a default burrow designation for new migrants. I still need to play with the system more, though - I've only got one crash-prone fort from v31.01.
Edit: Apparently, you can designate burrows in unmined rock. Thanks for the correction, Oglokoog.
-It's a large nuisance to personally set up rooms for each and every dwarf, building a coffer, door, and bed in each one. Maybe have a designation for the items that lets the dwarves lacking one claim and add the marked items to their room (won't work for the beds, of course, but that's difficult to solve.)
-Auto-canceling digging designations a royal pain 3/4 the time that turns a simple room designation into a few minutes of painful re-designating the same tiles. It's a good thing every now and then, but halting digging due to water/magma you already know about that won't even be accessed by the digging designation you selected should be redesigned.
-Linking levers could be done a lot better. Like by showing you which levers are linked to what when you select them.
-Having new migrants come in is a real pain, because there's a number of settings you want to assign to them that you either need to do right when they come, or you forget to do it and are confused about whether or not you assigned them properly. There are a number of ways to get around that (for example, setting up workshops to only allow dwarves of certain skill levels/identities or naming them custom professions) but the list is long. Not seeing what tasks your dwarves have enabled at a glance is another related problem.
-Not being able to see multiple creatures on a single tile is a fairly basic annoyance with the game's engine. It does have a decent workaround at the moment.
-Not seeing details about dwarves. This is the largest problem that I have with no graphics. It's not huge, but I'd like the UI a lot more if you could see what they were wearing/bleeding from, for example. But it's not worth the time to code, IMO.
Let's just say adventure mode has a large number of issues. I think a lot of them will be fixed by Toady's renewed attention to it over the next few months, though.
Macros solve a number of issues that I'd have, if I could figure out how to use them. I never played the 40d# releases heavily. Not so much a problem with the game as with my own laziness.
Hmm... to somewhat digress, I wonder if there should be a "needs screen", which goes over a simple list of things that your dwarves need that are color-coded/numbered with the amount that you have of them, like food/food production, dining rooms or bedrooms. Maybe add "desires" below it as well for less necessary stuff. That could really help the new players coming in, and occasionally the odd expert who forgets to include something basic in his latest fort.
There's still a lot of minor things that can be done. There are really few absolutely bad interfaces in games, but plenty of ones that have little annoyances that could be solved with a bit more attention by the developer. Dwarf Fortress's complexity makes for a host of those issues almost by default, thus it tends to have a relatively bad rap among games.