In my opinion, oblivion ruined quite a bit.
Can't cast that spell? wait a few minutes.
Have an awesome enchanted item? What's that, it's out of charges? You'll have to waste one of your valuable soul gems or pay massive ammounts of money to have it recharged!
In morrowind, magic was a tactical resource: You would always want to save that last bit. restore magica potions were VALUABLE.
Enchanting was a bit overpowered, but, at least when I played it, it was only castable, with no constant effects or on hit. On the other hand, you could prepare expensive powerful spells in advance, with an apropriate chance of failure during the creation, of course, and your hard efforts payed off in the end.
My opinion?
TES V should have magic gained only through sleep and potions(and spell effects like partial spell absorption and stealing it from opponents) and enchanted items should regain power, but more slowly than morrowind.
They should also revive the more freeform levitation, jump, lock, and slowfall effects while keeping the script effect(only good feature of oblivion!).
On that note, the physics in oblivion is only good for murdering the guards in their sleep and then dragging the bodies over each other in ugly suggestive poses. That and falling boulder traps.
What would be best is simply "items fall on slopes of greater than x degrees or if you took the items under it" and nothing more. Yeah, go make a floating candle in morrowind. Not that hard.
Morrowind had uniformly decent graphics and gameplay while oblivion wend for graphics only.
Get rid of voice actors entirely, at least for NPC mashups(everyone in city X says this, don't give them the same recording, stupid!) and maybe replace it with a good voice GENERATOR that takes in at least 20 character inputs, and another 10 for the dialog itself, allowing for emotions while staying consistant to the character's voice. This will also make modders VERY happy. Just remember to include an override or make the values script editable so people can recreate some stargate scenes or maybe GLaDOS.
In fact, if they did that, it might sell an additional million copies on that feature alone, more after the modding community have made entire new games off of the engine...
Skills giving bonuses every 25 is STUPID. I liked morrowind's spell chance that varied with fatigue, magic remaining, magic skill, and luck. It wasn't a "you have to be level 50 with destruction magic to use this" it was "you probably won't be able to use this until at least skill level 40, and it will take level 50 to have a decent success rate".
Although slight improvements in effect based on level were nice, in a way, the loss that had to happen for it was way too much.