Spoilers for Guild Wars follow, so if you haven't reached level 20 and Ascended yet (or aren't interested in the game), you might want to check out something else. About hamsters, maybe.
Guild Wars attempts a number of innovations in the MMOG genre, one a new business model that abandons monthly fees in favor of regular expansions. This idea, along with the game's heavy use of instancing, lends itself to a single narrative spanning your character's evolution from initial to final level.
The story of the game so far involves a land laid waste by otherworldly demons, refugees in need of protection, secret societies, betrayals. The final chapter sets you on a spiritual quest. After a number of trials, you face your final ordeal, a battle with your toughest enemy yet: yourself.
More precisely, your doppelganger. Bigger than you, with a glowing skull for a head, but otherwise your double right down to the skills you have.
Whatever you can do, so can it. And it hits harder. Prepare to die, when you face this cat at first. A lot.
To win, you need to strip away that which makes it, and you, strong. You must become weak, so your opponent will be likewise weak.
What this means in practice is somewhat unglamorous, basically loading up your character with abilities that are intended to wreak havoc on classes other than your own. You then whomp on your doppelganger with whatever blunt object you have at hand, while it does the same, and hope that your armor holds out.
But the overall concept of wading, after twenty hard-fought levels to become powerful, into a battle that can only be won if you are in effect naked, is compelling and fresh on both gameplay and story levels. I hope this is a sign of things to come from the game.