Before I say anything else, let me get this out of the way - what the hell is up with putting worthless replicas of high-end equipment in a display case with a very hard lock? Really, Oblivion, if you don't want me to have that stuff, just don't give me that stuff. If you think it's unbalanced, you could easily have put something less game-breaking inside that case. But getting my hopes up with a glimpse of awesome treasure, making me overcome a significant challenge, and then snatching it away like that? Total dick move.
Which is my roundabout way of saying that I really enjoyed the more stringent advancement requirements of the modified Thieves' Guild, but I wish I had a more diverse selection of stuff to steal. It's a little embarrassing to be the legendary thief responsible for Cyrodiil's unprecedented rash of flatware robberies.
The slower pace of the game also means that I have very little to show for my time playing. After 16 hours with this character, I have reached level 6 (and even that is misleading, because 3 of those levels I gained by casting the "Light" spell 250 times in a row, in the hopes that by grinding my Illusion skill enough to be able to cast "Chameleon", I would actually be able to practice my thievery in places that were not already abandoned). I was also, at the last minute, able to enter the Arcane University. It's a little annoying that it took so long to reach the meat of the Mage Guild storyline, but to be fair, the recommendation quest chain is also a bit of a slog in the base game.
I kind of regret parting ways with Oblivion so soon. It's probably the most divisive of the Elder Scrolls games, and one that many believe has not aged too well, but I enjoy it. I like how colorful and bright it is. The wilderness is beautifully lush, and the architecture of the various towns is, while more familiar than the fantastic cityscapes of Morrowind, nonetheless wonderfully designed and all-around attractive. My only real complaint (aside from the obvious one about level-scaling) is that the game is kind of stingy with mp, which makes higher-level wizards tough to play. And once I got into it, I really enjoyed the mods, though the basic needs mod was marred a little by the fact that food and alchemical ingredients are not distinguished by the base system, so scrolling through the menu for something to eat was kind of a pain.
Yet, I have to move on, because even after 20 hours, I have barely scratched the surface of the game. I haven't touched the main quest, I'm still only an apprentice wizard, and even the Thieves' Guild quests are only half done. And there are still at least 3 more major side-quest chains (Fighters' Guild, Dark Brotherhood, and Arena), two whole DLC expansions, and an uncounted number of minor side-quests. It's all a bit intimidating, but it wouldn't ordinarily dissuade me. It's just that, were I to do all that, I would then have to jump into a substantially similar, yet (in my opinion) superior game, and do it all again, and since I don't feel like I have a whole lot of unfinished business with Oblivion, I'd prefer to just jump right into Skyrim.