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 :)
An easily accessible list of all used tags would be handy.
A while back, I polled the community on the possibility of subreddits. Most people said they wanted them, and I said I'd investigate.
I talked to a couple of people and eventually ended up talking to Tricycle, the developers of this site. They told me about their own proposed solution to the community organization problem, which is this new Discussion section. They said that searching the discussion section by tag was equivalent to a sub-reddit. For example, if you want a sub-reddit on consciousness, the discussion consciousness tag search is an amazing imitation
I told them I wasn't entirely convinced by this and sent some reasons why, but I haven't heard back from them lately and I'm not going keep pursuing this and make a big deal of it unless a large percentage of the people who wanted sub-reddits are unsatisfied.