Most big things start out as jokes. (I don't hold an opinion one way or the other.) But you have to admit they are doing something semi-interesting.
Not some silly NodeJS/MongoDB API.
Meteor is fantastic and has changed the way we should develop web products forever, and more importantly has opened up a whole new set of expectations for what websites should be capable of. Websites are going to become living breathing creatures as a result. Meteor will lead to a truly collaborative realtime web--one where viewing any website is a group activity, rather than a private one.