The performance, I had frequent slowdowns in oblivion, but skyrim runs way better then oblivion on my comp and it also looks way better. Skyrim seems to have a very well optimized graphical engine

Crafting, definitely. It was a feature that was pretty long overdue for a TES game, and it looks like they took some inspiration from the Nehrim mod for Oblivion.

The crafting/smithing system. I find it so rewarding that I can go hunt, skin some pelts, turn those into leather, create some armor, and sell that or keep it for myself. Infact, I just spent 20 minutes creating an entire set of leather armor for myself.

I also love the higher difficulty. Its still possible to defeat mobs of enemies, but you have to be smart, skilled, and a little lucky to do it well.

Vampirism isn't a game-ender. It was way too hard to get rid of before.