Quick, not-fully-baked thought: Twitter's successful because it's a better place to have conversations than Facebook or Google+. It's public by default, so new people can join conversations. It doesn't have the fiddly UIs that Facebook and Google+ have for comments on posts (sometimes it feels like Facebook/Google+ let you have conversations in comments only grudgingly) partially because there is no distinction between a comment and a post.

I'm not really sure what role the 140 character limit plays here, if any. (It means that it's always easy to scan your feed without worrying that you're missing something, but that's more about the reading experience.) Keeps it flowing, keeps it about the conversation?