Planet13 - 8:02 pm on Jan 23, 2012 (gmt 0)
I think the letter to the editor idea is great.
I might take it a bit further and have an animated face of The Devil surrounded by flames and say something like, "Hate This Article? Let Us Know!"
Or maybe, "Flame Us Here," or "Go Ahead, Make My Day! Leave Us Some Feedback"
Something a little bit provocative.
What I have found that DOESN'T work (for my sites, anyway) is: "We would love to hear your thoughts, good or bad, about this article." That mostly generated spam comments from people whose username was "Cheap Flights To Kinshasa"