The following may be the clearest statement on this issue I have ever read:
Think about it. You’ve paid taxes. It was complicated. But did the existence of different tax brackets significantly contribute to the complexity? Of course not. What’s complicated is calculating your income (especially if you, like me, derive income from several different sources) and calculating which deductions and tax credits you’re entitled to claim.
Now remember that the Steve Forbes version of the flat tax would not tax certain kinds of capital income at all. Matthew is much better at explaining tax policy than Mr. Forbes!