I agree with your dislike of inline metadata as used with Twitter. However, that doesn't mean that we should exterminate the metadata entirely. We should instead hang it off the side. Hashtag-like things can exist just like blog entries with "topics" or other group-like tags. Similarly, the fact that one microblog entry is responding to another can be explicit as well (perhaps requiring some sort of adaptation of pingback/trackback).

Of course, once you get rid of Twitter's length limitation, these "microblog entries" can be as long as regular blog entries, and that's not a bad thing at all. It does mean that any dedicated clients you've got need to be smart about hiding some of the text (as, for example, Facebook does with its "more..." button). One nice feature is that you solve the annoying problem of comments. Today, you never know where to look for comments. Twitter @replies? Facebook comments? FriendFeed comments? Google Reader comments? Too many comments! Now, comments are just another kind of microblog entry (kinda like tweets) but you wouldn't have to see them if you weren't following the original conversation (kinda like just about everything else).