Thieves guild has some cool quests, more tailored for stealth than the normal game quests are. The Dark Brotherhood has some of the best characters in the game, and probably the most dramatic quest storyline.
I agree except for the best characters part. I found the mage to be really annoying, along with Babette. To me, the characters were all too over-the-top. Cicero was supposed to be, so I can forgive that one (he was actually really good. Annoying as hell, but good.)
No one really displayed cold, calculating professionalism that I would expect from an assassin. I miss Vicente Valtieri, Ocheeva, Teinaava...at least a certain speaker makes a return.
Quest line was pretty good. I would say really good if I wasn't put off by some dialogue.
Theives guild was good. A nice change from the robin hood antics from oblivion. Kind of a let down ending though.