I love the U-abhn in part for the dramatic reveals that you get. The best is coming up the Stephansplatz exit. You ride up an escalator, and then look up, straight up, and the spires of St Stephans reveal themselves, at first high, impossibly high and then you ascend to street level and it is still massive, but then there are some near human features. You can see the doors, and the ground for reference and you are not lost entirely within the heavens. You have been Revealed.
Donauinsel has another reveal. Especially in the whispy snow and grey sky which I had. You glide to a stop, get off the train, and then you are in the MIDDLE OF THE RIVER! Really. You walk a bit to Donauinsel, or to the Vienna International Center and all of the new buildings, but you are in nature in a primal sense.
Exploring a city is filled with connetions. Where you realize that the 'Naschmarkt' that everyone tells you to go to is really the same place you had been going to - you just didn't realize the name.
Stumbling onto Karlsplatz via Ubahn was one of these experiences for me. First, the station is great fun. It is massive, because there are entrances at each of a number of far corners.
The exit towards Karlsplatz is like an underground garage. You could drive cars right now a ramp and into the station. You stroll up and you are in a gorgeous park and in the middle of Vienna.
And there is another amazing church, and a Christmas market, somewhere on the other side of the station is the Naschmarkt, and the Secession space - which as near as I can tell is a hacker space for artists, largely supported because one of the group did something cool which is kept in the basement and spits out euros.