Elizabeth Lawley has a very interesting piece on how/why she blogs -- among many other things, she points to two features of blog writing: " the informality of lightweight publishing and the immediate distributed peer review and feedback". I like her term, "lightweight publishing" - because the power of the web is that potentially people can read your words, even if that's not why you got into it in the first place. And today (as compared with 8 or 10 yrs ago) it's easy and even sometimes free for anyone to create a nice-looking blog with very little effort, thanks to built-in templates etc. The significance of that is real -- when I first taught students HTML about 5 yrs ago in a writing class, you could see the thrill as they learned to create sites that looked good. This is an image culture after all.
"Lightweight" is often perjorative in our culture (which tends to value "deep", "meaty", or "strong" things) but it's perfect for this medium -- or at least a certain kind of writing I'm beginning to think of as blog writing -- the type I like to read, and hope at least sometimes to write. Something beyond "private journal writing" but not necessarily as argumentative, polished, or constrained as publishable academic writing tends to be. Inbetween, and basically ephemeral. Though as someone interested in the history of print ephemera, I'm glad the web leaves some traces. (and has its own archive!)