The comment capsule is intended for commenting on the merits of the hub itself or the writing skills of the author, so most people will address a comment along those guidelines. However, if the person who wrote the hub has a following, the comments may look more like friends running into each other at the mall. Comments will have little to do with the topic of the hub and much to do with catching up and ribbing one another.
By the same token, if the author and commenter have developed a friendship (or are related), that person's comment will likely have little to do with the topic. As marywanders said, it's "know your hubber".
If a particular comment put a hub in a different light that inspires you, by all means write your own hub on that insight! And spread the joy by mentioning in that hub who inspired it!