I didn't see any posts about the ability to tag posts or make comments under replies/posts.
Here is the case for why those are good features to have:
-When moderating a big ol' Stackexchange forum, I'd often just tell people "click on the --blah-- tag to see all the posts on --blah--". This is more reliable than using the search function for everything, which brings in a lot of random results that happen to use that word.
-You can have way more tags than categories, and as a forum grows there will be more and more user navigation done through tags.
-Tags just worked, really well, for Stackexchange.
-Commenting lowers the barrier for posting on a topic. If I have a minor comment I'd rather say under the reply "hey, good point, but have you considered x" or "here's another example of x" or "nice to see you Mr. X!", rather than make a whole new reply.
-Comments lead to other comments, which leads to cool comment threads under popular replies. This makes it a good mix of linear and branched commenting.
-Comments can be up and downvoted, which encourages participation even at small levels. Less commitment to a big reply with still some reward. Witty but short comments help conversation develop.