I think the Thieves' Guild questline suffers from really poor pacing and a criminal lack of thievery, which is part of the reason I think it is too short. You can blaze through it in an afternoon and the Radiant quests take all of 30 seconds to complete, offer no story, and are just go to x and do y repeated over and over.
The Thieves' Guild questline is indeed much longer than the Companions or College of Winterhold, but those suffer from even worse pacing problems so it's like saying someone is a better fighter than Nazeem. Sure that's true, but it's not saying much.
Personally, I think that the only faction that has any pacing whatsoever is the Dark Brotherhood's, and that goes a long way into making it my favorite of the factions in Skyrim.
Your character joining the faction should not be the inciting incident for ever major event each faction sees. You should have to do a couple of crap jobs before even dealing with the higher ups and have missions only tangentially related to their main plot available at regular intervals (and not just the same ones you had before, quests that actually expand on the characters you're dealing with).
I agree, and it's somewhat ridiculous that you sort of just waltz in the mages college for example, do a couple simple tasks and bam. You're the new archmage. Same for the others... I mean really, it doesn't make sense. These guilds are full of masters and they can't handle simple tasks of their own profession.
I enjoyed the Dark Brotherhood too, for example, but it would have made more sense if they sent the more senior members to handle, you know, the stuff I can't say because it would be a spoiler.
Just saying, it seems like there could have been more potential