1) Huge amounts of ways to play and customize your builds.
2) Unrivaled complexity in pretty much every aspect of the game
3) Graphics and audio are generally pretty good
4) Combat is fun and looks cool especially when playing in a party
5) Helpful community
6) It's free!
1) Not very Noob -friendly
2) The story and the dialogs aren't particularly memorable
3) Trade chat is a spam-fest
4) Item drops are disappointing 99.9% of the time. Trading will be your primary way of acquiring gear.