[Honclfibr]

A flat tax on long term capital gains (i.e. taxing them at the standard income rate) would remove the incentive for long term investment, discouraging retirement savings somewhat. It also removes the penalty for dumping stock options. It would lead to higher volatility in the stock market, which is a bad thing any way you look at it.

Besides, unless I'm mistaken most flat tax proposals called for the elimination of capital gains tax, not rolling it into the standard income rate. Under such a scenario you would see people like Theresa Heinz paying zero tax, not more.