(Bumped because of Comments)
I am set on my heels in the evaluation of "Darn Nice People." I have learned more than I am willing to share, but one important point: There is a continuum of underdog rooting that eventually gets to the pathological. Yet underdog rooting is in itself a good human quality. It is along this axis that part of the trouble lies.
As with most other sins, this is virtue out-of-control, unchecked. I am tempted to say that in half-measure, this would be good. But I have read enough Lewis (and Tolkien, Chesterton, Solzhenitsyn, Orwell, MacDonald) to suspect first that this is a quality that has evil in it down to the root, which only manifests as quantity increases. Dangerous territory, of course, because that is easily said about the virtue of conservatives as well. Subtle dangers cut to the bone.