Romney's tax returns showed he bank accounts in the Cayman Islands, the Bahamas, and Switzerland. It also showed Mittens was a hypocrite, because his campaign slogan was Believe in America, while he was parking his money offshore to avoid paying US income taxes. 

Trump's returns will probably show far worse, like the extent of his ties to Putin and the Russian oligarchs. That's why he won't release them.  The will also show he gives nothing to charity, and is not a very successful businessman.