In reality, the public option would be available, as one option among many other private options, to people shopping in the health insurance exchange. Any of those 30-or-so million consumers could buy into it. And with the exception of the fact that it will be a). not for profit, and b). accountable to the government (as opposed to to shareholders), the public option will have to behave exactly like a private insurance company--financed and stabilized by consumer premiums, without any subsidy from the federal government.
If something that has the support of 70 percent of the public can be considered controversial, it's because a minority of elected officials have charged that it's the seed for a single payer system. That's about it, though.