This guide is for anyone interested in making the best possible WordPress themes. Each public theme on WordPress.com is tested against these guidelines to ensure the highest quality. This includes security, plugin compatibility, and support for a wide variety of user-generated content.
The Theme Team at Automattic released this handy Theme Guide for WordPress.com-Themes which every theme is tested against before it gets released. I would love to see something like this grow into a complete best practices guide for themes, both for WordPress.com as well as WordPress.org