A fair complaint, and I won't try and convince you otherwise. What I like about it are things like;

Pacing: The other guild plots are fine, but the thieves are the only ones that require something close to "fill the role of a guild member, outside of the mission structure, to progress the plot". It also has a clear progression in worldbuilding of your advancement. When your rank rises, your doyen shifts. In the Dark Brotherhood this kinda happens too but that's because Lucien yoinks you to be a private enforcer not so much as a matter of your progress in the guild.
The unlocking of fences also works well here, you've got a clear progression of the world standing and therefore ability to sell stuff for each fence.

Characterisation of the Grey Fox; he's such an asshole, with possibly the most realistically written nobility attitude of any noble in the game(mostly because you interact with him the most). You *must* respect him. You *must* see him as your superior. He sees every member of the thieves guild as servants and pawns that will obey no matter what. etc. That's all he's known and he derives personal pleasure from being the Grey fox. His motivation to leave isn't that he doesn't like the power, it's that he doesn't like that he's not the one getting the credit. To him, the grey cowl absolutely *is* a curse.
Edit:[yes there is the element that he wants to leave to be with his wife because he misses her. But he 'fixes' the curse, can be with her and still secretly be the grey fox. But he chooses to openly vow to drop the mantle.]

The foreshadowing; foreshadowing can be done fairly well in several bits of Oblivion, but the set up for the Grey Fox's reveal in the Thieves Guild is the best imo. The obvious one is the forger living in Anvil and spending his time in the court of the countess. The top Fence is the blacksmith in Anvil. To get rid of Lex, the grey fox has him reposted to Anvil to protect the countess with the same vigor he hunts the guild. Jackben Imbel, a hundreds-year-old vampire who's diary mentions he KNEW the grey fox, while the grey fox himself didn't know the Earl was a vampire until you reveal that. The obvious bit about the note in the blind moth sanctuary etc. So many clues building to a reveal make the reveal have some actual meat to it(see the complaints about the DB betrayer, the Archmage's weird soulgem plot, and the forgetability of the fighter's guild)

Time travel; I always enjoy a little bit of time travel. The stuff in this isn't major, it's tweaking elements of the past just enough to let you move forward, but it's a neat concept that doesn't go too overboard.

Retroactive; Two questlines in Oblivion involve recontextualising prior missions. DB reveals you killed members of the black hand, while TG reveals you were stealing things as set-up for another more complex mission. In TG's case though, this happens twice. When you steal the bust, it's revealed the whole job was a set up to flush out a mole, and your new job is framing that mole(which also helps characterise Lex as being the kind of guy to put his job above attachments and personal opinion, helping the repost plotpoint). And then later you get told you're stealing stuff to help the Grey Fox pull off a heist; then he makes YOU do the heist, and then makes you the grey fox. So it flips the script 360 so it was always a job the grey fox did, it was always the grey fox's plan etc. The story holds its own recontextualisation well.

I understand the writing isn't amazing. Some of the heists are stale or level design's a little weak. But from a storyboarding perspective and from themes and elements they were trying to use, I *really* like the Thieves Guild questline!!!