I think to understand the UI you've got to have grown up with vim or emacs where offering a wide range of features via rather shallow keyboard interactions to enable power users. I wouldn't say that nested menu driven interactions are entirely suitable - and I think that the first time you go through it helps immensely to have a tutorial open that will lead you through some basic things (designate chopping some trees for lumber - for instance). I also think that people who have been with it longer have a leg up here since the UI has slowly accumulated more complexity over time - the military interface and trade interfaces in particular are pretty hard to grok.
That all said - I would say that DF doesn't fall into a trap very common in modern games: it never lies to you in the UI for simplicity, it is clear about what it's doing and most of the errors are relatively clear to comprehend. Some of these have been accumulated over time (i.e. damp/warm stone warnings) but generally when a feature is added that feature gets consistently signaled in all the UI portions that need it and information is sanely repeated. A good example is lava-safe metals which are pretty clearly highlighted in areas where it might be relevant (i.e. building a floodgate and switch mechanisms).
I've frequently heard talk that cell-shading adds artistic flair or bloom allows better tooling around creating bright scenes - but the immensely obvious and clear style that DF achieves brings precisely what it needs to bring to the table. Ascii graphics make that game clearly legible while still allowing the imagination to roam and beautiful structures to be created - making a multi-z-level dining hall with balconies adding extra floor space around the roaring fire in the middle of the hall with dwarves dancing merrily around it... that is quite possible in DF - it's just not going to look like a triple-A game.