Outrageously stable servers. I drop maybe once a day during peak hours if at all. I'll also note that there is no queue to get in like WoW. If there are too many people in one area you're placed in an overflow server to avoid crashing the server.
Great community. I've joked around with people online and had amazing conversations. There is the occasional troll, but those are rare. If you're lost there is probably someone in the same map with an answer. The ANet admins are actually active, so people who troll often, or have inappropriate names get hit by the banhammer fairly quick.
Combat mechanics are very smooth. You don't get many weapon skills unless you're an engineer, so it's more about learning how to use what you have more effectively. There is a dodge mechanic that helps move combat away from standing in place which will only get you killed here. If you are a mage be ready to dodge out of the way a lot, because monsters can hit you hard.
Good PvE. While I dislike the mob mechanic some of the enemies are downright nasty. Veteran monsters can be challenges even to people who are a good ten levels above the monster (not all stats downgrade with your level). For larger monsters you need to get people together and in some cases organized. For the most part you can solo the game with relative ease once you start allocating trait points and making better character builds, but you still might want a friend when a tougher veteran monster appears (I'm looking at you Veteran Raptor Bird!) or when going through some of the more challenging areas. I can't say much about the dungeons just because I haven't done that yet.
Well designed world. There are tons of places to explore, and not all of them are pointed out to you. There are jumping puzzles, hidden places, and quite a few hilarious NPCs that just aren't on the map. You'd have to work to find out the highly detailed lore of the game, but it's there if you go looking. It's also quite pretty.
It's by far one of the best MMOs I've played in a long time. WoW got a lot of things wrong, and in my opinion GW2 fixed most of them. If you like MMOs I can't suggest this enough.