I loved Morrowind, it was the first Elder Scrolls that I REALLY got into (I played the first two but didn't stick with them beyond a few hours.)

Just the persistence of the world was amazing. After I'd been playing for 30 or so hours, it really bugged me that my character had no place for himself. No house to store my loot, etc. So I came up with the idea, what if I kill some house owning schlub in town? Surely the game won't let me do that? The guards will come storming in and murder me, or the game will lock me out of the house, or the owner just gets magically respawned after a day or something, right?

Followed some dude into his house. Cut him down to shreds. Hid his body in a closet. Waited. No guards came. No respawn. Nothing. Satisfied, I took the key from him, and turned his house into my personal Daedric and Glass armor/weapon display museum.

I mean, we take this for granted now, but I don't think many games had done this kind of open world persistence before. (I think you could do it in Gothic.)

And then of course the layering of items. By the end I was a permanently flying/levitating, enchanted-out-the-ass Daedric armor wearing God. I killed Vivec because I didn't like his conceited belief that he was more powerful than me. I don't remember much of my fight with Dagoth Ur, other than that it was over very, very quickly, and I thought "Wow, you were what everyone was afraid of?"

I think it's that slow nerfing and eroding of the magic system and its abilities that I miss most of all from Morrowind versus the later Elder Scrolls.