1) My Oblivion install with graphic mods, notably Qarl's texture pack and MTAEVWD looks better then what I saw in the Skyrim demo
2) combat overhauls to Oblivion from Deadly Reflexes and Unnecessary Violence (created by amazing modders Skycaptain and Hex_Off) give more exciting combat then what I saw in E3 videos of Skyrim
3) Many mods today completely change the magic system in creative ways, much more then one system in vanilla Skyrim.
As for quests and the main story, meh, never was Bethesda's strong suite. No doubt the most exciting choice we will face in Skyrim will be if we respond to a quest giver with a "yes' or a "I'm not ready yet".
6 years ago I was stunned by Oblivion's graphics, delighted with it's open world but today I am seeing small improvements (no where near a Qarl's Textures) graphics and pretty much no change in the world design dare I say it, Skyrim is a re skinned Oblivion. So if a PC gamer can mod Oblivion into a better game then Skyrim why should said gamer run out on 11/11/11 and pick up his pre ordered copy? given it will be years if ever anything like todays mods for Oblivion are created for Skyrim
Please my fellow PC gamers give me and other forum readers your reasons for buying Skyrim on 11/11/11