When I am explaining what (I think) a blog is to folks who've never heard of one, a very typical question or observation is: "Oh, isn't that what a newsgroup or mailing list is?" I think one of the fundamental differences (maybe the only one) is that a newsgroup consists of postings by many people about a single topic. A blog, then, consists of postings by one person on his/her thoughts on many topics.
By the way: This is maybe why we need categories in blogs but not in newsgroups.
This harkens back to a discussion of what a blog is at all. For example, is the Dean blog is a blog, whether metafilter is a blog? There are other hybridgs. I happen to not think they qualify as blogs.
Net, net: It's a continuum - if two people contribute to a blog is different than if two thousand people contribute to it. I would say, though, that the more authors there are, the less of a blog it is.