"Given the choice between a Republican and someone who acts like a Republican, people will vote for the real Republican all the time." - Harry Truman
Obama's failed miserably at puting the immense political capital he had at the start of his administration, using the bully pulpit of the Presidency, reminding people early on of the crimes of the Cheney Administration, redressing the crimes of the Cheney years, restraining Homeland Security through the legal system, or passing effective healthcare reform (as opposed to the current corporate welfare bill), and just overall framing his administrative objectives coherently and early enough.
Obama was elected in a time of radical economic upheaval and radical demands for a solution - he's given typical DLCocrat half-measures that tend to, if more than anything else, strengthen the increasingly hereditary plutocracy of America. I don't care for the arguments that another Republican might be let in in 2012 if Obama's challenged, the truth is that Obama will pave the way for back-to-back Republican Presidencies if he is not forced to correct his failures. The Democratic Party needs a primary challenge to Obama immediately to correct his path.
I would like, in esscence, to see something like a "draft David Swanson for 2012 movement", not so much in lieu that he'd beat Obama as it would pressure Obama leftwards.
EDIT: Oh, and because of how poorly designed the Stimulus Plan was, I think there is a half-decent chance that Obama's depressed the labour vote for decades and may very well have cost the Demoratic Party the House for a decade as well. Just another thing to consider when contemplating whether a primary challenge against Obama is worth it.