Similar to thread flags and tags, I like it. Perhaps we can add filters, making it so some users don't see threads tagged with a tag they don't like, or tag posts older than three months, and tag official staff announcements.
I like sorting and hiding threads with specific tags, for example hiding threads with tag '[NSFW]'. If this is being implemented, just need a confirmation from theymos or a dev, then I can compile a list of tags whichever I know and can found in internet. This would decrease dev's work.
Flagging threads with '[INACTIVE]' automatically would be good too although there is a warning when replying to threads older than 5 months, IIRC.
-MZ