I appreciate being able to preview themes through the theme browser. The variety of the sample posts goes a long way towards preventing me downloading a theme only to find that it's not as nice as its thumbnail.
It would be more useful if the sample posts included a few more examples of common elements:
- Headings (within a post)
- Multiple comments on one post (to determine whether a theme does alternate backgrounds, etc.)
- Comments from people with Gravatars
- Comments by the post author
- A post with trackbacks
- A post with comments off but trackbacks on (and vice versa)
- Smileys (many themes try to style these as regular images and it looks odd)
- A post that uses the "more" tag
- Images in various alignments (e.g. right-aligned with wrapped text)
I have a set of posts that have most of these features (for my own development) and would be happy to pass it on if that's helpful.