No Aetna. No United. No Cedars-Sinai. Covered California's list of participating health care providers and insurers is out, and it's possibly more notable for who isn't partaking in the state's new health insurance exchange rather than for who is. The reasons for the big-name absences are manifold, ranging from payer strategy to high hospital prices to even accusations of gamesmanship. And they help explain the caution that -- despite news that premiums will be far lower than expected -- the exchange transition may be bumpier than some early, optimistic reports suggest.