What Are You Doing?
Does anyone even remember that question? That's the question that Twitter explicitly asks, and the purpose of any microblog. It's about putting original little thoughts out there that weren't worthy of big old blog entries.
Somewhere along the way, people discovered they could throw a question out there on Twitter, and people may post an answer, but noting a reply has remained awkward on Twitter. Other services, such as Jaiku and FriendFeed, have successfully made replying and commenting work nicely, but for some reason the population hasn't jumped, and they just stick with the same old awkward Twitter.
Twitter's fine as a microblog for original and independent thoughts, but I can't stand trying to follow a conversation. I've seen a feature request on FriendFeed to filter away @-replies when following Twitter, because we just don't want to see them and have to wonder what you're saying.
I still do like to comment on other people's microblogs (I'm a smart ass), but it better be easy to follow, because I'm not worth the trouble, and neither are most people.