What is a blog? Does it have to be social?

The other day I read an article that probably contained the worst description of a blog ever:

In a nutshell, a blog is a "do-it-yourself" website

Anyone who engages in the blogosphere would realise how rotten a nutshell this particular comment is, however the borders of what is and is not a blog are pretty vague. To me a key ingrediant of the blogosphere is the engagement with other users, unless we allow others to comment on our opinions blogs are little more than vanity publishing. Unfortunately it is not always a simple process to comment on some people's blogs, we can find ourselves hampered by blogs not allowing comments, having moderated comments, or having a comments section that is damned-near impossible to find!

Relating to the above, high ranking, linked-to examples:
-Yes, Seth Godin shows trackbacks, but I don't necessary want to add a blog post on a topic.
-I don't understand why Andrew Keen utilises moderated comments when he then passes spam and pointless insults for publication.
-Why does Dave Winer have comments when they are such a bugger to find?

Spam and offensive comments are probably more of a problem to high profile bloggers than me, although I was privileged enough to receive my own comment from a BNP idiot, and I can understand why they put safeguards in place. It is however bloody annoying to the fairly-respectable majority, who come across a comment to which they want to respond and can't.