What didn't McCain like in Romney's hidden tax returns?

Mitt Romney isn't against showing his tax return to repugicans. He just has a problem showing his tax return to the rest of us.
Remember, Romney handed over 23 years of returns to the McCain campaign when he was being considered as a possible vice presidential choice in 2008.  McCain's took a look at Romney's returns and decided to choose Sarah Palin instead (that has to say something, Sarah Palin was better than Romney's returns). Was the choice of Palin merely McCain's bad judgement or did McCain's people see a real problem with what they found in Romney's returns?

I am not a 0.01%-er like Romney, but I have done 'due diligence' for corporate acquisitions. If my employer or client was buying a company that someone else had passed on, I would want to have all the information the last potential buyer had before they backed out. Mitt Romney needs to come clean on what's hiding in his tax returns.