The thieves guild and dark brotherhood questlines are so much better than the others it's kind of ridiculous.
Really? I've started both, but:
Dark Brotherhood: The writing and voice acting for characters in the sanctuary is awful. Dialogue that an immature kid in grade school might write thinking it cool, voiced by actors that lack either skill or interest in the material (or perhaps who simply can't do much with the crap material). The armor isn't as cool as the DB armor in Morrowind or Oblivion. Astrid isn't as cool as Lachance. However, I haven't received the first major contract yet, just the three minor contracts. I intend to give the questline a chance, but I'm not impressed so far. Does it get better as it progresses?
*** minor, minimal spoilers about the first three TG quests below ***
Thieves Guild: This guild doesn't seem to be a guild for thieves but instead a mafia or mob organization. The first three quests involve framing someone, collecting "protection" money, and terrorizing a business into cooperation with a powerful noble (made powerful by using the guild to enforce a monopoly). Get better? Does the guild actually ask you to steal anything as the questline progresses? Extorting citizens and manipulating the economy is thievery, sure, but not the kind that interests me.