For what it's worth, my guess is that what Romney is hiding in his tax returns is not something that is clearly illegal or dishonest, but, simply the following reality:
Romney has made hundreds of millions of dollars
He has paid very little of that (on a percentage basis) in taxes
He has made hundreds of millions of dollars in part because he has structured most of his income in ways that enable him to pay the least amount of taxes possible
This "structuring" of income has likely taken full advantage of things like the ludicrous "carried interest" tax exemption that allows private-equity investors to pay capital gains taxes on income that is actually fees [This tax treatment is one of the most outrageous and unfair elements in the entire tax code. There is no logical basis for it, and it benefits only the richest people in the country.]
This "structuring" has also likely taken advantage of offshore accounts, the contribution of hard-to-value securities at low valuations to Romney's IRA (whereupon they exploded in value), and other sophisticated tools. These tools are, theoretically, available to anyone, but, in practice, are available only to those with tens of thousands of dollars to spend every year on tax-and-estate planning.
This structuring, which (let's be honest) is done primarily to avoid paying taxes, will look bad to most Americans, who will know instinctively that it's done to avoid paying taxes and that it's not something they will ever be able to afford to do--and, therefore, will seem unfair.
In short, by not releasing his returns, I think Romney is trying to avoid calling more attention to the fact that he is a card-carrying member of the 0.01%, a group of Americans who deserve to be proud of their success but who are also understandably viewed with suspicion and frustration by most other Americans right now, especially when they argue that they're still paying too much in taxes.
Read more: http://www.businessinsider.com/what-romney-is-hiding-2012-7#ixzz20hbsMRgP