Truly Engrossing.
After WoW, I played a few other MMOs for a month or so; City of Heroes, Everquest 2, and then I tried GW: Prophecies. I think at this point, I was frustrated in general with MMOs altogether that I didn't really give GW: Prophecies a real chance. Yes I enjoyed it, but my thoughts of WoW seemed to overshadow the enjoyment. I stopped playing MMOs for about 12 months, and just recently got back into the scene, started up Prophecies again.
I was immediately re-captivated by everything Prophecies has to offer. The environment is truly beautiful to just look at. The storyline is enjoyable. I have a few minor gripes with the gameplay (no jumping... no running off cliffs), but aside from those few minor gripes, it is wonderful.
Beyond that, the true enjoyment of the game is the opposite of what turned me away from WoW. Your progression through the game, and into PvP isn't based entirely on how many raids you've gone through and how many nice sets you've completed. You're limited to 8 skills to use. Knowledge of your class, and how to counter-attack a class you're fighting reigns supreme. Sure this is true with every game to an extent, but I believe that the Guild Wars series truly takes this to the next level.
Guild Wars: Nightfall is a wonderful continuation of the campaign. I'm already completely immersed in the new story. The gameplay is not only the same, but there have been improvements that were made for Nightfall that have been applied retroactively to the previous releases as well. The heroes really feel like customizable henchmen, which I enjoy. The graphics and scenery is wonderful as usual, and the sound is perfect. I think my interpretation of great sound for a videogame is when you just don't notice it--Not because it's not worth noticing, but because it just seems to melt seamlessly into the actual gameplay that you treat it all as one.
All that being said, it would be almost rhetorical to ask if this game is for everyone. Obviously it's not, and it's not your typical MMO either. Obviously it's more of a diablo-style game, where the towns are the common places, and everything beyond that is instanced. It doesn't do it for everyone, but for me, it's not only great, it's ideal. To me, this is the epitome of what an MMO should be. Hats off to you Guild Wars, another wonderful addition.