Comments are what makes a blog interactive, and lets a blogger communicate and interact with his/her readers. Comments are also a good way to analyze a blog. How is that? You can obviously gauge the popularity of a blog by looking at the number of comments per blog post. Its well known that of the many reading a blog post, only a select few respond to it in the form of a comment. 

For e.g., if the average number of comments per blog post is less than 0.5, the blog is relatively new or doesn’t keep the interest alive for its denizens. 

Blogs with an average of between 0.5 to 1 comments per post are barely getting it right with the readers or are evoking interest.
If it averages between 1 to 3 comments per post, the blog is doing fairly ok.
Between 3 to 5 comments per post, the blog is doing well, and has a stable reader base.
Between 5 to 10 comments per post, the blog is doing extremely well, and is usually regarded with a lot of respect in the blogosphere.
Anything above 10 comments per post, means either the blog is in the Top 50 or the blogger is a celebrity.

A good blog/blogger usually responds to most comments, and encourages more participation. After all, a blog post comments is a valuable feedback tool for any blog.