[salmonellaeater]

The problems with brigading, and in general using downvotes for manipulating what other people see, can be mitigated by doing personalized ranking (which Twitter already does - this is just a tweak). For your personalized ranking, the downvotes of people similar to you would count much more than those from people different from you. So if a group comes in and brigades a thread, their votes won't do anything to your view if they're not similar to you.

This opens a question, though, of whether the person who started a thread should have any control over the conversation other people see. In the extreme, it could be almost like those browser extensions where your group has a parallel conversation about a website, unrelated to the comments on the site itself. In the other extreme, the OP of a thread is curator of their personal walled garden, where their enemies' replies are all auto-hidden.