Plurk is a new microblogging service that support threaded conversations. Darren Rowse pretty much nailed the differences between Twitter and Plurk:

The responses that emerged on Twitter were a whole lot of individuals responding to me in isolation. Your followers on Twitter don’t know what other people have answered [...] but the problem is I was the only person who saw them ALL.

and,

10 minutes after asking the question on Twitter my ‘replies’ page contained all kinds of messages. Some were still responding to the first question I asked, some to the followup, others were responding to earlier tweets, some had moved on to new topics with me.

Twitter was not designed with conversations in mind, as a result, you can’t have a long conversation without being lost.

This is a major flaw of the service (at least a fundamental difference). That said, Twitter remains of a great value and a good source of breaking-news.