It's a social and sociable take on the problem RSS was intended to revolutionize but hasn't really delivered on. Its only ideological conceit, of 140 character messages, actually simplifies the user experience and keeps the "communities" that form there on the rails. "Chat" is arguably one of the "stickiest" and most compelling applications of the Internet, and Twitter delivers a chat-like experience with huge numbers of people that manages to be both non-ephemeral (IM and IRC conversations disappear forever as soon as they end) and (therefore) discoverable.

I don't believe for a second that Twitter realized any of this when they started; like so many good things on the Internet, Twitter's benefits are emergent, not planned.

Wave is what you get when you try to catalog all the possible benefits of Twitter and plan a product to deliver those benefits.