@Viktoria Michaelis -

Even non-commercial themes need to ‘sell’ so a good deal of improvement is desired to gain more downloads. We can’t all download a plain theme and work on it to see what the creator should have shown us in the first place.

But that's the point: for the vast majority of what you listed, the Theme developers don't have control for the Theme directory preview.

Theme developers can define default header and background images, but I think that's unnecessary. If a Theme uses the "custom-header" or "custom-background" filter tags, you can trust that the Theme supports the core implementation of those features, and thus that you can add your own custom header or background images.

Featured images are difficult to handle on a generic preview/demo server, also, because custom featured image sizes must be regenerated on a Theme-by-Theme basis (something that I assume would currently be prohibitively difficult for the Theme directory preview server).

So, there's really nothing that the Theme developer can do to improve things with respect to the Theme directory preview.