David A. Kennedy reports that ThemeShaper has created a, “new set of requirements for all themes on WordPress.com to follow, geared toward making themes easier for people to set up and use.” The list also makes theme switching a better experience.
This list of “theme user-experience requirements”, or TUX, covers items like layout standards, menus, and pagination. These are guidelines for WordPress.com themes, but of course many of these guidelines would (and probably should) apply to any publicly distributed theme.