Well, let's start with the insurance companies themselves. Those aren't big capital? And what about the banks they consort with in a web of mutual reliance? And then there's pharma, which would suddenly have to negotiate prices rather than writing its own inflated ticket. Then there's the class war point Doug makes, the weapon that health care--and the threat of its loss--constitutes in every workplace that still provides it, to keep workers in line, to avoid organizing and strikes, and generally to make workers dependent on their jobs for not just their livelihoods but in some cases their lives.
Setting aside their general class interests, the only 'big' capital interests that might see some payoff are manufacturers with strong unions which have been able to force the cost burden onto the employer. See a whole lot of those around these days?
In fact, I'd say that single-payer would be a more general injury to the interests of big capital overall than financial re-regulation would, since re-regulation really only curtails the range of motion of one sector, and might help others.
Jenny Brown