There’s one practical, fun way of becoming a better WordPress theme developer and WordPress.org theme reviewer at the same time. Have you heard of the doingitwrong theme?
It’s a theme where anything you shouldn’t do in a theme gets done. The WordPress.org theme review team has made it available as a learning resource: doing it wrong teaches you how to do it right.
You can learn more on how to use the doingitwrong theme on my latest article for SitePoint.