"Skyrim's UI is much better than Oblivion's. And with mods, it's not even a contest."
No, it's really not. Play it again. Skyrim's UI is more consolfied than Oblivion's. So much scrolling. Another huge annoyance is that all the prompts in the game have to be manually clicked "Really quit Alchemy? etc)", you can't simply hit enter or space to quickly leave menus (in Oblivion's case: tab). And yes, it can be fixed with mods. But I don't really consider that a valid point, anything can be fixed with mods. We should be focusing on the base game, because really both games are great with mods.
"It's true that Morrowind's NPCs sucked, but Oblivion's are worse: instead of making meaningfully unique NPCs, I ask a guard to tell me about the Emperor, and all he says is "he will be sorely missed". What the fuck, I wanted to know about the emperor, not how you feel about his death."
Maybe he's a dumbass that doesn't know much about the emperor? If you want to know more about the emperor ask someone who's in a position of power or close to the emperor (the blades?) or read a book. You can't expect all NPCS to have wiki-tier info on things like Morrowind because 1) ultimately it's unrealistic and 2) it's boring (and lazy game design) for the NPCs to say the same things over and over again. And yes, before you respond, Oblivion does have NPCs that say the exact same things, and maybe there is less actually text, but ultimately they were both done in a very lazy way (well, it's Bethesda so...). Morrowind just has more text (because it's so reused) so it seems like more effort was put into it than actually was. I think fully voiced NPCS was a step in the right direction for the type of game Elder Scrolls tries to be, it adds to the world's realism and adds an additional layer of immersion.
Oblivion's NPCs actually feel like residents of the world they're in as well. Most have complex schedules. Breakfast, lunch, dinner, daily activities, many even travel the whole map to chill with friends in another city. NPC shopkeepers have schedules too, and actually close up shop. They feel like they're actually living in the world, compared to Morrowind which literally has NPCs standing in the middle of town 24/7, shops never close, etc. Why does Morrowind even have beds? No one sleeps. Oblivion is much better in this regard, and it's better than even Skyrim's NPCs too I'd argue, which are almost as bad as Morrowind's, while some have minor schedules there's nothing in Skyrim that compares to Oblivion in the NPC department, it's a total downgrade.
So while Morrowind may have more text than Oblivion, quality is not always equal to quanity. And that's not the only factor deteriming the quality of the NPCS either.
"It takes away from the RPG aspect of the game when it's your player skill that makes the real difference."
No it doesn't.
Most RPGs actually rely on player skill for everything, you just think they don't because it's hidden behind stats and the like.
And your point only applies to lock-picking, which is still incredibly hard at low skill levels if you don't know what you're doing. Disposition caps out if your skill level is too low, so speechcraft is still dictated by your skill level. Like I mentioned earlier, in Morrowind you simply bribe people to raise disposition (and this does not have a cap) or just click intimidate over and over again. It's shit.
"You can choose not to exploit Morrowind's system. In Oblivion, there's no logical reason not to take advantage of it: Magicka simply regenerates and from an in-game point of view, it makes sense to keep healing yourself. In Morrowind, there's no logic to acting like a narcoleptic all the time."
I get your point, but you can choose in Oblivion too. Pick Atronach, lol. Or simply don't fuck around waiting for magicka to regen (the developers didn't intend for you to run in circles for 2 minutes waiting for your bar to regen in the same way they didn't intend for you to abuse the rest system or quicksave feature in either game). If you're using a freewill argument then I can too. This was a game mechanic choice by the developers, and it's not necessarily any better or worse than Morrowind's, ultimately it comes down to personal preference. The magic regen point, while I responded to it, is not really a huge problem. Go play Oblivion again and see how far waiting on magicka to regen gets you when you're surrendered by 3 ogres early on, and good luck. It only becomes "broken" when you're high level and your intelligence and wisdom scores are close to max (when it takes about 30 seconds to fully regen all your magicka despite your total magicka score). At that point in Morrowind you're already plowing through absolutely everything, even with the difficulty slider at max (I know this for a fact because I am currently replaying Morrowind and I'm heaving a very easy time at max difficulty). Morrowind is easy as fuck, much easier than Oblivion (especially if you're not min-maxing). Access to OP items early on, alchemy, enchanting, etc. Most of the mechanics in Morrowind actually end up breaking the game's challenge.
"In Oblivion, they went for generic high-fantasy in the most generic way possible. You can make generic look good, but Oblivion simply looks awful, it's greenery everywhere."
Generic? Yes, I addmited this. But you not liking it is an opinion. Morrowind is not 100% unique either, and like I mentioned before, people need to stop pretending that it is. Oblivion looked amazing when it came out. Morrowind, base game, is brown with fog everywhere. Technical limitations, I know, but vanilla Morrowind looks like utter shit compared to vanilla Oblivion. Concept art doesn't really matter when it's not accurately portrayed in the game.
"I really liked Vvardenfell"
Me too but I don't let that blind me to the problems of the game. That being said, Oblivion has major problems too. They're both imperfect.
I think one of the most legitimate points against Oblivion is the leveling problem, although you're probably not familiar with that since you admittedly played very little of the actual game.