One danger a public plan could face is that it becomes the dumping ground for high-risk patients, suffers higher costs, raises rates, loses members, and basically replicates the process by which non-profit, community-rated Blue Cross/Blue Shield got destroyed by HMOs, transforming the Blues into the for-profit monsters they (mostly?) are today. h/t to Matt Yglesias, who flagged this important article by Paul Starr:

http://www.prospect.org/cs/articles?article=perils_of_the_public_plan

Good reading, and we need to make sure we get what we need, not just any public plan.