From Flash Coder’s Etiquette:
Each email is a request for the time of other people… they have to spend at least some time with it, even if it’s just to delete your message unread. If you post too much, without offering corresponding value for the reading, then you risk diminishing the community, and risk getting yourself killfiled in the process.
That may be a good test for blogs, as well. I wonder how many of my posts qualify.