I don't care how many square miles they claim Oblivion was, it was TINY. There just wasn't as much stuff and not as much variety and not as many places. I think there were fewer quests too. Leveled enemies wouldn't have bothered me that much if they hadn't of leveled loot as well. In Morrowind, treasures actually felt like, well, treasures, whereas in Oblivion I'd hold off gathering anything unique until it would be leveled as high as it goes. I loved the epic loot stashes you accumulated in Morrowind, but Oblivion never managed to strike that hoarder compulsion in me.
I hated the simplified spell system. A lot of people bitched about how in Morrowind you could ALWAYS miss with physical attacks if you sucked enough, and I didn't like that either. I did like that they fixed physical combat in Oblivion, but then they made spellcasting simpler and stupider as well. I damn near smashed my controller when I found out "You must be at least X level to cast this spell". If they left the original Morrowind magicka system alone, I'd consider Oblivion superior for combat and spellcasting.
They dumbed down and simplified armor and weapons in Oblivion. I don't care if some things like spears were pretty much unused in Morrowind, I'd sometimes play a spear focused Argonian with medium armor just because I can. All classes in Oblivion seemed to play too similar (Between now equally effective physical combat and magical combat, and fewer choices for each), I felt like I could get more character variety in Morrowind. And then they added "perks" for mastering certain skills, where by the time you mastered both light and heavy armor, both glass and daedric armor offered no difference in protective value or weight (Light gets +50% value to equal daedric, while heavy armor gets weight reduction). AUGHAUHSDIOAUHSDA! WHY EVEN BOTHER HAVING MULTIPLE CLASSES!?!?
Bartering and inventory windows in Morrowind were superior. Oblivion's was tailored and cut down for console.
I liked the bit of the new alchemy system that narrows down your ingredients to those that would actually have an effect. Saves some time (though I just memorized formulas in Morrowind).
I think text was superior to Oblivion's voice actors. It really limited how much and what they could say.
And of course I miss my master of alteration who could leap through half a dozen loading screens and slowfall at the last second. That crazy guy that falls to death right outside Seyda-Neen who was convinced leaping was the travel of the future? He was absolutely right, he just forgot the ring of slowfall.
I could go on forever about Morrowind Vs Oblivion and where Bethesda went wrong. In a nut shell, Morrowind's gameplay had some considerably more oldschool and hardcore D&D gameplay roots compared to Oblivion.
...Oh right, the new game. I won't even buy it unless it:
-Has as dizzying an array of available weapons and armor types AND unique accompanying skills. They don't even have to be perfectly balanced.
-Has the superflexible Morrowind spell system, but with Oblivion's quick cast. Visibly regenerating magicka is fine, just as long as it's slower than Oblivion's game balance breaking regen rate (Wizards are supposed to run out of spells. I shouldn't HAVE to play with the Antronatch make a magicka based character interesting to play. They might as well have given regenerating arrows to archers).
-Morrowind's bartering interface, or better.
-Oblivion's physical combat (and poisonable arrows, which actually made marksmen worthwhile besides the ranged sneak bonus).
-Keep the creature and loot leveling limited to random travel encounters when it's been peaceful for too long. Otherwise, I like to be surprised by significantly stronger and weaker encounters.
-Has jump, levitation, and of course slowfall spells.
-Is at least as diverse and it's points of interest as dense as Morrowind.
-Has more than one voice actor per gender per race.
-NPCs have as much to say as in Morrowind, regardless of the lines being spoken rather than read.
-Generic NPCs have more than three lines or dialog options.
-As many if not more named/artifact items as in Morrowind, and possibly a museum that collects them like in Tribunal.
-Piques the treasure hoarder within me.
-Significant support, story options, and unique stuff for Vampire and Werewolf characters.
-Significant support, story options, and unique stuff for each race, and gender.
-Enough sidequests that it's physically impossible to beat all of them like Morrowind came close to.
-More guild quests.
-etc
...I've actually run out of free time to post, but I have more stuff I could list. I need to corner a Bethesda employee at a bar and rant at him until he calls the police. My prediction? It will suck.
But it will probably look really really nice and control well on a gamepad.