The more I play this, the more I think this is what Fable wanted to be.
This is the greatest open ended rpg ever IMO. If you like MMORPGs for the depth and size but you don't exactly like the MMO part or fee, then this game is for you. The world does evolve as you progress and the npcs reactions of you change as you accomplish more. You can do whatever you want. You do not have to do any quests at all if you don't want to, and just go explore the huge world. I know that I will be playing this game for months to come with many different characters because there is so much depth and the way you play changes so much with a different type of character.
The combat is awesome. You actually make contact and do damage when you attack and can block attacks. Arrows stick in the enemy after shot and can be picked back up. Magic is fun to use. Sneak attacks are awesome. The AI is great, especially in combat.
There are a few drawbacks, but only a few. Of course in a game this massive there will be some glitches and bugs. There are a few game stopping glitches, but if you save regularly this shouldn't be a big deal. Just keep many different save files. The frame rate will also stutter and there is a lot of loading. Although the load times are not long, they will irritate a few people. Because this game is so amazingly huge, beautiful, and most importantly fun, I couldn't take more than one point away for those flaws.
This is a must buy for any rpg fan, or even action/adventure/dungeon crawler/hacknslash fan. It is so addictive and you could very well be playing it until ESV comes out.