Making income tax a flate rate would stop exemptions, and EVERYONE would pay. For example if income tax was 10%... If you made $10, then Government gets $1, if you make $1 million, the Government gets $100,000.
You could do the same thing with a Federal Sales Tax to replace income tax. Every purchase in America would contribute to the U.S. Government's budget. This would work even better than the flat rate, by virtually eliminating tax fraud. Also, for those Amercians who don't want to pay a federal tax, they can simply not purchase anything!