I like how it's so community-oriented, instead of being like a game for your DS or something. Everything is about communicating and making friends instead of leveling up and such like it is on other MMO's.
Aside from that, I like how it's free to play, though you have the option of donating by buying cash. Of course, I agree with everyone else here: constant updates are awesome. Especially the Cygnus Knights. ;)
The things in life you really need to know:
You are not cool.
You are not smart.
Nobody cares about your great idea.
Nobody cares if you just got to level 675836268327892.
No matter what anyone says, this is the truth.