I thought Oblivion graphics were stunning on my graphics card (pc) but it's been a while since I played it. Maybe I'll re-install it and give it a try!
As for the combat? Not a great deal of difference really. One big improvement in Skyrim was dropping that silly agility skill other than that I still think (from an RPG point of view) that Oblivion is the better game. Certainly the mods represented a big difference from the ones currently available in Skyrim in terms of how much extra enjoyment and eye candy they generated. Player homes/estates, animation, races/classes, armour, to name but a few. Also worthy of mention was/is Apaachi's huge mod for clothing/armour weapons etc
Must admit that the ENB 3d modification does considerably improve Skyrim's general look for depth of field vision, water, landscapes sfx and so forth for me. Apart from some mooted points about perception of reality the only thing about Skyrim landscapes is their ethereal quality and majesty when looking at stuff like mountain ranges craggy valleys etc. I don't think that is translatable into Tamriel generally (possible exception of the Jerral mountains and Bruma) A big improvement is in castle, cave and cavern interiors which get better in Dawnguard . Those would represent an improvement in the the original look of Oblivion, but I think it a marginal one. Oblivion improvements? a bit more variety in what's on the other side of an Oblivion Gate
I agree jevo. For the quests Skyrim graphics would make a difference as the castles/caves etc could stand some individuality and custom interiors so good in Skyrim, but what is possible now with more sophisticated GPU's would not have been achievable in the heydays of Oblivion. The big thing about Oblivion quests v Skyrim quests is the poor choice(s) available to the PC in Skyrim conversations - sorry that's not poor choices just sloppy finishing. Even if you say no you still get the quest in your journal. That's not conventional rpg'ing