Wide open play that keeps you interested for a 100 hours or more.
Character development can sometimes be a distraction but not here. The idea of creating a character molded in your gameplay style allows for flexibility and reward to sticking to your plan by leveling up through the use of your major skills. I played the game as a "blade mage" so I relied heavily on destruction magic and a single-handed sword to dispatch enemies. It worked very well and the game never because too difficult, even in the middle stages where some of my spells were becoming a little weak against stronger foes.
The main quest is engrossing and takes you through much of the world. However, I think the game really shines in other quests you receive through interactions with NPCs. You can pretty much play the game for almost 20-30 hours without making very much progress in the main quest. This open style of play allows you take things as quickly or slowly as you'd like. The addition of new quests through Live downloads only enhances the replayability of this title.
If you enjoy RPG games, you'd be doing yourself a terrible disservice by not picking up this title. How good is it, really? Good enough to buy an Xbox 360 just for this game alone!