"What is Mitt Romney hiding?" isn't the most damaging thing that can be said about his refusal to release his tax returns. Here's what is: Romney thinks the rules don't apply to him.
Americans don't resent wealth. They resent a rigged system. Ordinary people have to play by the rules, but the lesson of the financial meltdown is that plutocrats don't.
One of the rules of running for president is that you have to release your tax returns. It's not a law, but that doesn't mean it's not a rule. There's no law that a presidential candidate has to debate, either, but that's the way we do it in America.
Romney reeks of entitlement. He thinks it's up to him to decide whether his financial life should be transparent. It doesn't even occur to him that he owes this to voters -- that it's an obligation, not an option.
He may or may not be hiding something. But it almost doesn't matter whether he ultimately discloses his tax returns, because he's already disclosed his arrogance.
UPDATE: It's the You People, Stupid.