Trump's whole spin on his acumen as a business man. Because of that, he attempts to break down every topic to a financial transaction of some nature. Sure that makes sense for trade agreements and domestic business etc. But it does not apply, no matter how hard he may try, to race relations and foreign policy and national security.
That being said, if you're going to hang your hat on being the world's best business man, then you're going to want to back that up with proof -- some sort of empirical evidence. Which to me, would be his tax returns. His reluctance (and ignorance) on not releasing them really bothers me. By not releasing them, it only makes me and millions of others speculate he is hiding something -- especially when he falls back on the excuse that he "can't" due to being under audit. Add on top of that the little "that'd make me smart" comment last night about not paying any taxes and I really have doubts about his money situation.
As a business man who certainly has made billion dollar deals all over the globe for decades, I want to know if he is indebted to any foreign countries/citizens before he'd become POTUS. That is a potentially huge conflict of interest. Not to mention his reluctance to put his companies into a blind trust if he were to take the oval office.