In parallel with the Theme Directory reimplementation effort being chronicled by @obenland, work is also underway on a new Plugin Directory theme. In a similar fashion, the goal is to bring the directory more into sync with the in-dash experience introduced in WordPress 4.0.
Unlike the Theme Directory, the Plugin Directory is not yet undergoing a backend reimplementation simultaneous to its visual refresh. As such, the change is more superficial, a modification of the existing bbPress 1 theme rather than becoming a true WordPress theme.
The plan is to first reimplement the simpler Theme Directory, transitioning it from bbPress 1 to WordPress. Then, with that experience and ideally extending on code common to both directories, we’d perform the same reimplementation for the Plugin Directory.
In the course of bringing the backends into WordPress and building on a shared base, we’ll be in a better position to implement long asked for features — particularly for the Theme Directory, such as: supporting multiple theme authors (#27-meta), readme.txt support (#215-meta), favoriting themes (#28-meta), changelogs (#45-meta), and just generally parity with features found in the Plugin Directory. And, of course, having the code become modernized and publicly accessible will facilitate additional future enhancements.