I agree that Oblivion had its flaws, but there at least had to be one thing you liked about it. It could be anything from a new monster to an appearence make over, an upgraded/new system, a new mod, etc. etc.
I'm comparing Oblivion to Morrowind(because I never played any of the other games)
For me... Havok engine(bad as it was it was than Morrowinds Morrowinds lack of)
-LOD, That is seeing distant land, trees and cities
-Facegen (it was hard to use, but few games have a system like it)
-The menu and Journal were a bit more Organized.
-Spider Daedra were cool
-Controled blocking was nice
-Combat system was nice
-Stealth was REALLY good
-Sigil Stones were cool
-Oblivion gates were cool
-I like the new looks of the Atronachs
-Level perks were cool
-CHEESE! For everyone!
-I liked the new look of Ebony arnour and weapons
-Providing Amber and Madness ore to the smiths in new Sheoth.
-Being able to enter a Deadric Realm(two or is it four?)
-Easier to find the deadric shrines
-The modding community
-Larger landmass
-The shivering Ilses themselves had a very interesting look.
-The new creatures introduced in the Shivering Ilses(I hope some of them appear in a new TES)
-Traps were more like traps.
-I know people don't like them but, the lockpicking and Speachcraft minigames(I was really good at them)
-Patrick Stewart(Thanks to Hamsmagoo for reminding me)
-Sean Bean
-Poisons
-Arena
-Radiant AI and all its systems(It could be better, but at least its their)
-Bows, the aesthetics of the arrows sticking into things and being less Character Skill based
-Waiting an Hour healed fully
-Buyable and Upgradable houses
-Horses and horse armour
Theres more, but I can't think of any at the moment