I think things are fine as they are - if anything, the "discussion" section may be "too cut off" from the main page, to the point where I could imagine them splitting in different communities. I don't think that's very likely, but I still wouldn't want more sub-splitting.

Tags don't seem like a sufficient imitation of sub-reddits (they don't feel like 'sub-communities', and I don't think most posters keep a list in mind of what tags exist and to what tag they should post), but since I personally don't see the need for sub-reddits except on reddit itself, I don't mind :)