Open Threads, Media Threads, Rationality Diaries -- these are so obviously different from the remaining posts, and they already have a strong tradition, so I believe there should be one subreddit for them (for all the repeating threads together).
More meta: The subreddits should reflect a difference that already exists. One should be able to move the existing articles into the new subreddits. (Moving them would be useful to show readers what exactly is expected where. I don't expect admins to move thousands of articles, just perhaps the last month to create the momentum.) For example, if you have an idea for a new subreddit, but you can't list articles from the last two months that would belong there, then this subreddit most likely shouldn't be created. So as a sanity check, before really creating the new subreddits, we should list specific articles from the last month or two which should be moved to each of them. (Unless we implement the topics using tags/keywords.)
Less Wrong is based on reddit code, which means we can create subreddits with relative ease.
Right now we have two subreddits, Main and Discussion. These are distinguished not by subject matter, but by whether a post is the type of thing that might be promoted to the front page or not (e.g. a meetup announcement, or a particularly well-composed and useful post).
As a result, almost everything is published to Discussion, and thus it is difficult for busy people to follow only the subjects they care about. More people will be able to engage if we split things into topic-specific subreddits, and make it easy to follow only what they care about.
To make it easier for people to follow only what they care about, we're building the code for a Dashboard thingie.
But we also need to figure out which subreddits to create, and we'd like community feedback about that.
We'll probably start small, with just 1-5 new subreddits.
Below are some initial ideas, to get the conversation started.
Idea 1
Idea 2