There are a lot of mechanical quality-of-life changes that GW2 made that are different from the standard MMO. You can deposit crafting collectibles into your "bank" from any point in the world, and then you can view just those items when you go to a crafting station. There's no breath meter when underwater - underwater combat actually even has its own automatic item loadout for weapons that is applied. There are warp stations everywhere (which sadly means no mounts, but it's way easier to get around). Also, your level scales down to zones beneath your current player level, so you can group with a friend and do quests at a lower level without steamrolling everything.
GW2 has an exceedingly impressive world. The cities you go into actually feel like cities. That was one of my main complaints about WoW, the world was actually quite small and the outposts and towns you visited always gave the impression through dialogue and events that the populations involved were much bigger than the physical reality of it would indicate. GW2 seems to be much more massive in world size and I like that.
Since I'm a UI guy, I've got to recommend GW2's UI as one of the prettiest I've seen.
Lastly - and perhaps most importantly - GW2 makes you feel like you're part of a larger story. You have the requisite side quests, but you have a primary story questline for *your character* specifically through most of the game levels. This is something WoW kind of did in Vanilla with specific class quest chains, but many of these were self-contained and didn't pertain to the larger world story.