Tidying up the user permissions section, so that permissions can be managed properly through a UI. (RESTful API comes later, as per my earlier discussion on same.)
Working on the part of the plugin system that allows data to be saved for a given plugin on a site, blog, or systemwide level. This was another one of those things that was originally far more complicated than it needed to be. Sometimes the simple approach really is the best one.
Laying some groundwork for how themes will eventually be able to incorporate settings and code more elegantly. Right now it's all ad-hoc, not aesthetic at all.
Refactoring a bunch of stuff involving the way certain DB helper functions were off in their own libraries instead of integrated into the DB model definitions where they belonged. E.g., when saving a new user, the verification routines that the user data is valid is now part of the model itself. I originally didn't have this because I had some cock-eyed notion that it didn't really belong there, but now I know better. Sigh.
Category management is also in and working, if not very elegant. It can look good later.
Theme-specific control panels and settings can wait until after the first formal release.
Tag archives, a feature I was holding off on, will have to be in the first release after all, from the look of it. That's partly for the sake of supporting some of my own use cases, and partly for the sake of making it easier to do certain other near-term things (in short, implementing it constitutes a problem I'd rather solve sooner than later).