@Felix -

And I could say that a theme meant for download should be at least somewhat generic and make the best out of any content it’s given.

That's why, as part of the standard Theme review process, all Themes are run through the Theme Unit Test data - to ensure that they handle all normal use cases, as well as many edge cases.

Many themes eschew standard WP mechanisms in favor of custom code that has to be specifically catered to, and that’s wrong.

All Themes hosted in the official Theme directory are required to incorporate the core implementation of any included features.

But the official demo content is also very bland; lack of featured images on posts (to use your example) is an obvious omission that would be easy to fix.

Supporting featured images wouldn't be as easy or trivial as you imagine. The Theme directory preview server uses the same instance to serve previews, and changes the Theme "on the fly". But custom image sizes, as are normally used for featured images, must be regenerated on a Theme-by-Theme basis. In order to display featured images properly, the preview would likely need to have a unique instance for every hosted Theme, and would have to regenerate intermediate image sizes every time a Theme is updated.

(Though, I'm sure Otto could come up with something that would work much better.)