Since he won't release his tax returns, and does admit they'd be politically damaging, all we can do is guess.
This is one in a series of speculative diaries wondering what could possibly be worth the price he is paying by remaining secretive.
In honor of the Olympics, today's guess is about tax write-offs, such as the $75k write-off Willard and Ann took for a dressage horse. I imagine that examination of 12 years of returns would show many other examples of deductions which are not illegal, but also not available to 99.99% of American tax payers.
And raising that question is a threat not only to Romney's campaign, but to his entire entitled class.