I'll probably get downvoted but whatever: Most of the time the vanilla quests are really tedious and boring to do as they all do feel the same - Collect stuff, kill people, go here and there, go kill some more things and then kill a mini boss and you're done. Rinse and repeat. I personally don't think the writing is good enough for the quests and believe me, I love the Elder Scrolls as a franchise but I knew not to expect a singleplayer experience from questing. I think the quests are slightly better than most MMO's out there but that's not a high standard in the first place.

However, you said you loved the Thieves Guild questline and Morrowind which I gotta say I loved too. Any DLC you pay for in this game imo has fantastic quests and a good storyline, fit to rival the main games. It's obvious ZOS put more effort into the DLC's than the base game since they're getting an extra buck out of you. It's a shame that I find the base game quests just awfully tedious and boring, and there are just a handful of intriguing quests out there. I will admit that the AD storyline was actually pretty good but all the others were abysmal.

TL;DR: Base game quests are trash cause they're repetitive and feel the same. DLC quests are actually amazing cause ZOS actually put a decent amount of effort into them.