Looks like Mike Arrington's started a nice series of niche/vertical overviews and is publishing them every Sunday.
Assumed criteria: Categories that the web is (or continues to) game change; categories that combine structured and unstructured data, categories with large revenue.
Last week was online dating; this week it's TV-watching sites.

Mike's round up of TV sites is well done and well-articulated--this is a series worth looking for.

(And one I hope Mike continues--I'd like to see coverage not only of the well-known travel and autos categories, but an overview of the health space.)