Well, there are many great MMO games out there and a few (not many but a few) that I consider overall better than WoW. Of course, it is really just personal opinion, WoW was made for casual players - that's why it has such a huge fanbase. My mom plays WoW, for crying out loud.
It's the Wii of MMO's, where it has sold more than other MMOs, but it's made for casual MMORPG players/gamers, that's why so many people get into it, even non-gamers.
GW2 is going to be pretty great as well from what I have seen, but it just doesn't scream anything new or fresh that hasn't already been done more than a few times.
There have been many Diablo clones, and all of them have failed to live up to D2 (although many were fun and enjoyable in their own right), it's just simply great to have the King of the genre back again, and I'm just so ready and pumped for it.