Another problem in having themes *only* available in the repositories is the matter of screenshots. While apps, for the most part, can be described with an icon and a text description, themes, obviously, cannot. It is entirely necessary to have at least one, but maybe several screenshots, so that you can see how it will look before you bother downloading and installing it.

This is where a site like n900themes.com would work out awesome. Maybe even set it up to link to the .install link for the themes in the repos?