Elder scrolls.
The fact that you could actually have more variety in weapons and class, etc. It was like, "wow, I can use melee, with blade, axes, or hammers! Or bows! Or magics that have various elements and sub-classes!"
In Fallout, it was more "Wow, I can use guns or melee! And, uh... VATS? And... mines that nobody ever uses?"
If they just improved the voice acting and the graphics in Oblivion along with fixing that first STUPID quest in the Knights of the Nine expansion, I would buy it straight away. Another point, is that Elder Scrolls' combat can gradually get more and more complex, with better dodges, types of strikes, magic, and the such, while Fallout is just shooting.