Other MMOs I've played, I never ever recognized anybody else. Here it's like... wow, so many people I know, and even more I know of. Makes me feel like I'm part of something awesome.
I love lots - the magical feeling evoked when wandering through the illustrative lands, the fact that I can levitate, the sounds of tincturing... most of all though, I have a close knit group of friends in here whom I actually adore - and one in particular has fast become one of my real life best friends.
Basically, I found glitch when I needed an escape from some serious life stress, and have fallen in love with everything about it.
Apart from making string and wall segments - which can go bite me. ;-)
Everything. Especially the community of wonderful people -- and, as diave said, the ability to run into friends on the street -- but also the art, and the story/mythology, and the element of exploring, and the nonviolence and spirit of helpfulness, and the amazing devs, and the amount people care about the world (yes, the arguments in the forums are in many ways evidence of a good thing), and the varied gameplay, and the ability to choose what you want to do, and the excitement of changes and updates to the game, and... Everything.