I’m not sure quite what it will really be good for yet, but this Comments API proposal sounds cool anyway. Comments and an API are enough for me: I’ll worry about what it’s good for later.
The one problem I’ve got in terms of doing a dotcomments or Movable Type implementation is the lack of anything to use as a comment author identifier. Apps that don’t require a login to comment (like virtually everything I’ve ever used, except for playing around with Manila), end up getting a variety of names and email addresses for a single comment author. Retrieving all of Shannon’s comments around here would require a good memory and capi.getCommentAuthorIDByRegexp().