That's nice, but the thumbnails appear to be of the site layouts, rather than thumbnail metadata, which would seem to make them less than useful since it's not contextually relevant. Most of what's visible is the header of the page (which is repeated in the link) and some thumbnails are even blank.

That said, having tried to make a HN clone that did use metadata for thumbnails, even that seemed not to add value in most cases. Turning a URL into a useful thumbnail seems to be kind of a hard problem.