Literally the game will just keep going, and going, and going. And going. And going. There's just -too much- to do. Although there is the occasional repetition and tedium in the questage, you're always traveling somewhere, unraveling something, getting new skills, and finding/buying new equipment. The auction house is one of the most well-organized trading systems I've seen.
The world is also HUGE, just enormous, and immensely detailed, every place having a story, every background rendered beautifully. The people and the places all have -character- to them. Going into Stormwind on foot is breathtaking the first time. And it's even more awesome if you know it from previous WCs. I remember flying by Blackrock Spire on my way to Ironforge, then by the Altar of Storms, just completely in awe.
That's kind of the problem. The game is too good. It's like a DRUG. I know if I buy it, my life will be sucked away -_-