Let me start by saying what a fucking improvement.
The Thieves Guild in Oblivion was literally 3 hours of fetch quests, finally culminating into the big heist in the White Gold Tower where the Grey Fox passes his mantle onto you in a heartwarming conclusion. The Thieves Guild in Skyim is about 4 hours of anything but that.
The quest chain starts off rather unassumingly, when a character named Brynjolf asks you to try to frame a local Dark Elf on stealing a ring from an Argonian's jewelry shop. I thought this was just going to be another mundane quest for a local, like sabotaging the light house in Solitude for that one Argonian, so imagine my surprise when I realized this was what was left of the Thieves Guild.
The Thieves Guild was never much. In Oblivion it was run out of people posted around Cyrodil, and in Morrowind it was run out of some bars around the provinces. I was in heaven when I realized the Thieves Guild finally had some sort of a guild hall, and I was totally prepared to make it my own. After beating it, I learned that eventually I can restore it to the former glory it once had, eventually acquiring a blacksmith or an alchemist (I'll make sure to report back when I get all those things). Way to keep the game going long after the story is over, Bethesda.
The entire structure of the Thieves Guild quests are completely different from the last ones. Instead of sneaking into houses to steal some guy's journal or change the tax ledger in a guard tower, the missions are long and arduous stealth sections that seem to have come out of Splinter Cell. The mission where you sneak around the East Empire Trading Company's warehouse is by far the coolest stealth sequence I have ever done, and it's in an RPG. That's a job well done.
I've been having a major problem with Skyrim in that there just hasn't been much epic loot from quest chains to keep me saying "wow!", and fuckkkkkk did that change with the end of the Thieves Guild. The Nightingale Armor is one of the coolest pieces of armor designed by Bethesda, totally playing on a look for an agent of shadow.
In closing, Skyrim once again fails to disappoint me when it comes to improvements over its predecessors. GOTYAY.