I try to avoid reflexive blogging about blogging, but I was trying to explain the significance of blogs to Stuart at lunch and this struck me afterwards.
The thing that makes blogs different from email is citation. I can paste URLs into email, but they are cumbersome, and launch the browser. With a tool like blogger you can read something on the web, add a comment and link to it in your blog straightaway. It is much easier than putting it in an email, especially for the reader, and it keeps a permanant record for you (or others) online. Once you get into it, you find that you read other people's blogs, and comment on them, and link to them as well. One interesting thing is that these make them very Google-friendly, so that increasingly if you search for a hot topic, a blog will have made it to the top because of these mutually-reinforcing links. (eg searching for 'trustworthy computing' gives you someone's blog). It is fulfilling the original concept of hyperlinked text on the web.
A blog is also great place to keep your �sprit d'escalier bon mots while waiting for an occasion to reuse them.