It depends what the goal is. If we want "simple," then taxing all income (wages, Cap gains, dividends, etc) at the same rate is very simple, and that is appealing. It also strikes most people as "fair"--no matter how you earned it, once you get it you pay the same tax rate on it. There are at least a couple of problems in implementation:
Cap Gains: If you bought an asset 20 years ago and sell it today for 10% more dollars than you paid, you really didn't earn 10% in income. Due to inflation and the decreased values of today's dollars, you really had a tremendous loss --of approximately 36%. So, to truly tax the value of the income, any Cap Gain tax should be indexed to inflation. That increases complexity (I can see the voter's eyes glazing over). In addition, a significant portion of the cap gains of most stocks is due to retained earnings--the stock went up in price because the company held back some earnings to invest in the company. Those earnings were already taxed when the company earned them, to tax them again when the stock is sold violates the principle of fairness for which we are striving.
Equitable treatment of capital: Is there any inherent reason that income produced by putting your money at risk should be taxed at the same rate as income produced by your labor? After all, you are taking a risk. Will the government treat the results of your risk equitably--treating losses just as they treat gains? So, if you put $1000 at risk and earn $100, they take $10, but if you lose $100 ("earn a negative $100") they pay you $10? I don't think so.
Practical impact on the economy: The US has had the same tax rate on wages as it had on investment income (capital gains, interest, dividends, etc) several times in our history. For example, the 1986 Tax Reform Act made the cap gains rate the same as the rate for earned income (up to the 28% rate). The impact of this policy is (frequently) that business slows down as capital availability dries up. In 1997 President Clinton signed another piece of tax reform legislation lowering the top cap gains rate to 20%, and this had a positive impact on the economy.
So, in some cases, the appearance of fairness really isn't, and in some cases simplicity brings neither equity nor prosperity.
But I still think the tax code can be very simple, fair, and promote prosperity (largely by turning the tremendous energies now devoted to "gaming the system" of our bizarre tax code to producing things instead)