In contrast to the inflammatory exchanges on some of the news sites which I frequent, the comments on our blog posts are usually pretty tame (for which I am thankful). However, I am surprised when people comment on really old blog posts (yesterday it was an 18 months old post) as if anyone is likely to see or care about the comment. This is OK on the Forum, but not a good idea on the Blog.
Here's the difference: on the Forum when you add a reply to a topic, that topic jumps to the top of the list in that forum, as well as appearing in Today's Posts and New Posts. Thus anyone browsing the Forums is likely at least to scan the title and be aware of your reply.
The Blog, in contrast, is strictly linear. While you can search it for keywords, you cannot look for new comments. Only David and I are notified of each comment posted (and we do try to reply where necessary). Thus the comments to older posts are essentially buried, and there is not a lot of point in adding to them, or continuing a discussion on any post over a month old.
The other advantage to the Forum is that any member can start a new topic, whereas this Blog is more personal, and mainly written by David (when he has time). ;-)
I hope this helps to explain the difference between these sections of our website. Your comments are welcome on both....