For the anonymity thing, look at my other comment[1]
Tags/categories may seem like a fair idea, but what i don't like in UI's is information overload. So keeping info on a discussion would be pretty cool, but make sure i don't see it if i don't WANT to see it. it should be available, but not "in-my-face".
Also, what do you think people will react to having to type tags or subjects for their posts?
You know what would be super amazing? A discussion visualization/organizing tool. That could perform an analysis on the comments and perhaps group them together by relevance/chaining/sentiment. Would be cool if i open a 200+ comment page on HN or anywhere an immediately see a visual representation of what's going on. What aspects are being discussed and in what sentiment. Like someone posts a Product Landing page. I open the comments and see a lot of discussion about "UI" or "security" or "usability" or "heath risk" or "privacy" , each marked with +ve or -ve sentiments. Now THAT is something worth working on.
EDIT: the tool could do something as simple as marking keywords and giving posts a red or green BG with varying shades based on the sentiment...
[1] https://news.ycombinator.com/item?id=6933982