Sub-reddits strike me as unlikely to work well with a community of LW's size and activity; I can already imagine the deserted areas whose last thread was made 6 months ago. The discussion area seems to be working well.

On the other hand, I'm not sure tags are performing their full potential as organizational aids, with people forgetting them and different words and conjugations for the same concept and all. Having a modest selection of recommended or enforced tags might help (if enforced, there being an "other" option for anything that doesn't fit).