Joining the Reviewers Team is simple, just a matter of getting your environment setup (4-6 minutes) and grabbing a theme for review. The most interesting and valuable part is not that you’re helping out the community, but that you’re learning yourself. I personally reviewed a little less than 20 themes (and still counting) but I learned so much about them and the best practices of developing them, the Settings API, coding standards, and damn I learned two functions i had no clue about (checked() and selected()) — hope that doesn’t make me a rookie ;)
Konstantin Kovshenin from Introducing Minimal Georgia for WordPress, emphasis mine.
The WordPress.org Themes are reviewed by community members. As Konstantin points out, not only does this help the community as a whole, he also learned something new as part of the process.