I propose that we bundle the Sandbox theme with WordPress.
Sandbox is a theme which is loaded with semantic CSS selectors, making it possible to write beautiful WordPress themes with only modifying CSS. In addition, it is already widgets-friendly, and is licensed under the GPL, making it perfectly compatible with WordPress in legal terms.
There is a great deal of support for the theme out on the wp-hackers list. Bundling Sandbox has come up a few times, with general support from all directions, but nothing has really happened.
In addition to bundling Sandbox, I propose, for style.css-only themes, that Sandbox is made the default template. (e.g. without having to specify /* Template: sandbox */)
The rationale for this is that Sandbox is much easier to theme with just CSS, and is much more useful than using the Kubrick template, which doesn't contain as many selectors.