Just one thing I worry about this idea of theming in the ranking of sites. When I do a search, I want the most relevant *pages* that match my search. If I do a search on "purple peguins", I don't care if a page about purple penguins happens to be on a site that is almost entirely about green walruses. While a search engine based on themes will find well the sites that best deal with the content, this could result in missing important pages because they are off theme to the site they are on. I can see theming being particularly bad for "potpourri" type sites, which have a lot of content about a lot of unrelated stuff on the same domain. A lot of personal type sites come to mind here. The person just has a lot of different content that interests them, but it isn't really related.