What is the difference between a blog and a newsgroup?

Recently in a conversation I was asked to comment on what really was the big difference between a blog and a newsgroup.
After all they are both a reverse chronological list of postings, right?

My answer, maybe obvious, is this: a newsgroup is (typically) focused on a certain subject matter, and has many different people posting their comments and questions on that topic. So you follow a newsgroup because you are interested in a specific subject matter.

A blog is (typically) focused on a certain author’s voice, and often covers many different topics, of interest to the author. So you follow a blog because you are interested in a particular author’s point of view.

Of course in reality it’s not quite that black and white, and there are shades of grey in between.