It's only happened to me a few times, but I loooooove it when a couple of commenters on the site discuss their differing opinions of my comics or what they think will happen, without me being included. I always feel like I've "made it" in some way when that happens!
I generally try to reply to all comments to let people know how much I appreciate them, but when I'm reading comments on other comics I often find it more interesting if there's a community of fans talking amongst themselves.
Have you ever had mini conversations on your comics? Do you have any ideas for how to encourage them?