Oldie, but Goldie. By Dave Winer. From 2003, but still fresh.
Rather than saying “I know it when I see it” I wanted to list all the known features of weblog software, but more important, get to the heart of what a weblog is, and how a weblog is different from a Wiki, or a news site managed with software like Vignette or Interwoven. I draw from my experience developing and using weblog software (Manila, Radio UserLand) and using competitive products such as Blogger and Movable Type.