So, I've just started replaying Morrowind having had Skyrim fatigue.

I can't believe how good this game is. I had completely forgotten having played Skyrim since release.

How dark the nights are
The journal
Spellcraft
No compass - so you have to figure out for yourself where to go and find things without it being pointed out to you.
Degrading weapons.
Good guilds.

That and a bunch more. Feel free to add your own but yeah, I had forgotten how much deeper it is.