crossing between TWO separate bbPress installs
Three, actually. Themes can be reviewed too. :)
Also, Scott wrote the vast majority of the review code system. I just offered input and minor bugfixes.
Essentially, we have a bbPress install here for the forums and another one for the plugins and themes directory. When you look at the Support or Reviews page on a plugin, you're not really in the plugins directory anymore. You're looking at a custom "view" of the support forums, showing you topics relevant to the plugin and either in the Plugins or Reviews forum. The topics use a custom taxonomy to mark what plugin/theme they are for. When you make a new support-post/review from that view, then it gets automatically marked as belonging to that plugin/theme via that custom taxonomy. The ratings are tied to the plugin, and user, and review post, all via metadata. Stuff like that plugin taxonomy is also used to allow plugin authors to do such things as mark support topics about their plugin as "resolved" as well, which other people don't have the power to do.
It's all custom code because of the site's overall architecture. Not something easy to package up and release for general purpose use.