Your participation would go up if your second round was by poll, too.
Too much hassle. If someone can't type out a short response because they are afraid people will disagree, their loss.
Hassle?
Polls count votes for you and aren't any harder to post in the first place. Maybe you don't want to do it that way, and that's fine, but it is a much easier way of running these kinds of things.
...and I don't think people don't post only because they're afraid of disagreement. I doubt lurkers are simply afraid of their opinion being criticized, they just have a much different relationship with our community than those of us that post regularly.