I played WoW for nearly 4 years, I've raided everything in the game up to the sunwell. I was addicted.
I stopped around the turn of this year and i was then invited into the general beta.
This game moves the genre away from the generic WoW style mmo and into a sort of misty future.
The game's combat mechanics are next gen. I know that phrase gets thrown around a lot, but it really is. Combat is exiting and always changing. The fatalities are great fun to watch, extremely good fun to execute.
The graphics are beautiful, the environments, the towns and cities, the character models and the overall feel. Its very mature, decapitated bodies, bodies on spikes, heads on spikes, blood, guts, it doesn't hold back.
Monsters don't feel randomly placed, they feel like they're there for a reason. The adaptive shields of the mobs are really great fun, it keeps you constantly on your toes and you cant just spam buttons.
The negatives are the mass instancing of the zones. At first it used to bug me badly and annoy me, but they've reduced loads times from 1-3 mins down to 1-10 seconds, depending on factors. With the instancing it allows you to do a single player style side to the game where it feels like your impacting on the world.
Without sounding completely biased, if this game doesn't succeed, then there wont be another mmo with as much potential as this in a very long time.