I like to imagine there's a 1 in 10 rule. One person writes what ten others are thinking. Maybe it's not true, but it helps me get a picture.
There are some blogs that try to stick to a single topic. I think that's a good strategy, but I don't use it, primarily because I don't think there are very many people out there with my exact interests.
So, this is in direct opposition to my first statement. I'm unique, but no one else is. Silly, right?