I read quite a few blogs and I think it's clear that all good ones have a theme. For a blog to be interesting to more than your closest friends and family, it has to be either insanely funny and well written, or it has to be focused around a specific subject.
A great example of someone who does both is Steve Yegge. He has a witty and entertaining style, and his posts usually have something to do with programming languages. It's a blog every programmer should read.
Same goes for Kathy Sierras posts on Creating Passionate Users is also an absolute must to read. She is also great at using informative diagrams and nice pictures. That's generally something most blogs are missing. Humans like pictures.
Another thing with blogs is how often people post. Steve Yegge used to post rarely, maybe one post per month, but instead it would be a huge 15 page post. Paul Kedrosky's Infectious Greed is an example of the opposite. His posts are usually brief, but it can be as many as 20 posts per day. Point is though, they both stick to a few selected topics and don't waste time writing about things that aren't interesting.
There are many more blogs I should mention, but I'll link to them in posts on topics that concern them instead of just dumping a link here. Or, maybe that's what I should do?