Having taken a look at quite a few of the top 100 blogs (based on linkage) I have to ask the question - "Why Blog? What is the motivation for some of this blogging."
Guru Blogs which are written by someone with deep interest and/or knowledge in a particular subject area. Joel on Software
Collaborative Blogs - several like minded individuals gather together to comment on some particular area such as technology, politics or the media. e.g MetaFilter
Aspiring Writer Blogs - I'm not too crazy about this categorization because I can't define it very well.
e.g. Halley's Comments
Social Blogs which are generally one person's running commentary on their day to day life.
[ I gotta flesh this out with more examples and links ]