As a disclaimer before I start: This is a more technical article that deals not so much with story or graphics, but with game design and mechanics. If you've been following Guild Wars 2 for some time, you won't really find anything new here, but maybe you'll want to show your friends who aren't normally into MMOs.
I know there have been a lot of “Why Guild Wars 2 Matters” articles already. They all talk about how Guild Wars 2 is changing the rules of the MMO, throwing the tired old stuff out the window, making the genre feel fresh again. That's cool and all, but mechanical innovation is not the not the most important reason to play Guild Wars 2. It's the design philosophy as a whole.
Guild Wars 2 may well be the first MMORPG where YOU, the player, actually matter.
In a standard MMO, you don't matter. Everyone is the prophesied hero destined to save the world. Everyone does the same quests. No one has any measurable effect on the world. MMORPGs seem to have forgotten the meaning of “RPG” — roleplaying game, where you make choices that have an effect on the world and story. Guild Wars 2 hopes to change that.