Eh, I noticed a lot of things in Fallout 3 that people modded into Oblivion. Lights changing throughout the day, items being able to be combined into something useful(See: Midas Magicka and the Fallout 3 workbench), facial hair, eyeglasses, more body slots(Seriously, check the GECK. There's more that can be added to the body then in Oblivion for sure. Not to mention3 addon slots that aren't labeled), housing that you DON'T have to buy, useful house upgrades, unarmed weapons.... a lot of things they cut out and they learned that they shouldn't have.
Oblivion was often critisized for having too much emptiness. Fallout 3, there's barely emptiness. There is also less redundant caves and ruins to dive into as well, most every one seems unique in some way. So really, the modding community seems to have given them ideas, or vice-versa. Interesting