Subreddits seem to be working pretty well, and we've been creating a lot of language subreddits lately. Languages are easy because they basically exist separate from the regular reddit. The topical reddits, however, are a little more delicate.
We've been watching the subreddit request and feature suggest reddits and have a good idea of how we're going to approach the other subreddits. The key thing to know is that we'll try to avoid creating subreddits for a specific topic per se, but rather we'll create them for a specific group of people. For example, a developer reddit for people interested in programming and the likes.
There are certainly cases where a topical reddit is warranted (music, sports, science, politics, etc), and we'll get these in due time. However, an important point to remember is that subreddits are not tags. We'll likely implement tagging later as a method of organizing your saved sites and categorizing links in finer detail, and these will coexist with subreddits. The distinction between tags and groups (subreddits) often becomes fuzzy, and we even had to write on our whiteboard about a month ago "groups are for people, tags are for topics" so we wouldn't lose focus.
Anyway, stay tuned, we should have some more non-language-based subreddits up very soon. We just need to figures out a few other features (cross-posting, aggregating, etc) first. A major priority during the process is to make sure we don't take away anything from the current reddit. We want to provide a home for the links that may of interest to some group of people, but not necessarily to the reddit community at large.
