I'm trying an experiment (and am way behind schedule) at /r/MKaTS and /r/MKaTH along these lines. There's a private and a public subreddit, one for more closed discussion, one for more open. The idea is to build these out.

Using flair, you can get something like the related-subtopic discussion. See /r/dredmorbius (a solo bloggy effort) or any of the big subs with flaired discussion (/r/AskHistorians or /r/AskScience) for examples -- you can look at the full sub, or dive into a specific flair's topics.

A significant problem with Reddit is that establishing these structures is difficult. Setting up post flair -- the names, the styles, the sidebar search, etc. -- is a major PITA. FSM help you should you want to revise the scheme later.

And you're still stuck with the problem that it's not possible to filter out a flair to report only posts above some arbitrary cutoff (you can sort by "best" or "top"), not that the moderation system gives you any particularly good mechanism for doing that in the first place.

Reddit (as with many discussion systems) is a bit too focused on the now and not sufficiently on the good. I'm particularly annoyed that it's not possible to revisit old posts for discussion (the six month comment freeze), a feature of G+ which actually turned out to be really useful.

There's also the whole Notifications dynamic which ... simply doesn't work well. Yes, you see if someone's mentioned your name, specifically, but you can't get a general notification of discussion on a post (unless you've specifically subscribed to it, and that only for 48 hours). That's utterly unworkable for larger discussions, but works well for small ones.