There will, no doubt, be many embarrassing things we find out when Romney finally releases his tax returns: How much he's really worth, where his investment income really comes from, where his accounts are, maybe even some enterprising journalist will find some things that weren't reported (like more off-shore accounts and business holdings). But the real damage will come from the 1999 - 2002 tax returns that show Romney reporting Bain and Bain affiliates income as "Active" investments rather than "Passive". Active income is taxed at a lower rate - Passive income is taxed at a higher rate.
Romney has insisted that he "retired" in February 1999 - any Bain income after that date should have been reported as "Passive" income.
Ron Paul supporters should demand the tax returns be released before the Republican Convention, so the a different nominee can be chosen if Romney has been hiding the truth.