What about increasing the data density by eliminating the unnecessary minor WordPress version selections? There is almost never any reason that a Plugin would stop working from one minor version to the next, so why split up compatibility data between, e.g. 3.6 and 3.6.1?

This would not only provide better data, but would also encourage more participation.

Likewise: the "Tested Up To" tag should align with major versions. (In the rare case that a minor version causes a Plugin incompatibility, the "Tested Up To" (and "Requires") tag could list the minor version.)

I also think opt-in integration with the WP-Admin (even if via oAuth) is critical to widespread adoption. So much of the Theme/Plugin/core experience is built into the Admin, that I would imagine that most people never actually go to wordpress.org/themes or wordpress.org/plugins anymore. If true, I doubt that most people would do so just to hunt down all of their active Plugins, to rate or provide compatibility data.

With oAuth connectivity to wordpress.org, any active Plugin could be reported as "compatible" (with Plugin version and WP version, since they're already included in the update API), and it would probably be fairly trivial to hook into the Plugin deactivation, and add a "Report compatibility issue" checkbox that would log the Plugin as incompatible, with Plugin and WP version. And with oAuth, I would think that someone could write a support forum topic during the same process.

(Ratings could use the same approach, as an added benefit.)