I like being able to see the installation documentation also. That way I will know what plugins and various other steps I will need to do to get the site up and running. I hate not be able to see the theme documentation until after purchasing the theme.

For example. I was using a certain templates from one theme author and a plugin was used to create thumbnails on the homepage (auto generated). The new theme just bought from another theme maker used custom fields for homepage images. I did not know custom fields for the thumb process was needed because all the documentation was hidden away until after buying the actual theme. In the long run the theme has now been collecting dust because I could not be bothered editing 500 posts to add a custom field.

I also like to know if a theme will be updated for future versions of WordPress.