I think that comments define a blog. But I don’t think that all web site should have comments. The problem is “What is a blog?”. The format of a blog can be anything from a bunch of text written usually in spoken form, to a tutorial. Sometimes they are by date. Other times organised by category. Some are even so highly organised they are like a Wiki. People publish information all the time and lots of that does not need or want comments.
Some web sites (e.g. Freshmeat or CPAN) should have comments for modules software - but they are not a blog.
So I think it is fair to day that a blog is a publication which allows comments.
Another comparison is a forum. The first or Topic post is the main post and then people can reply - this is another common form of what we think of as blog.
Now the hard part - if I want to put up my site and allow people to read my opinions, tutorials or other ‘stuff’, then for people to understand it I would need to call it my Blog.
Blog has become the word for “your opinions home page”. Should all of those sites have comments - nope.
So tricky…