Mitt Romney pays less than 15% because the bulk of his income is from capital gains, which are only taxed at 15%. Once you add in deductions, it obviously goes below 15%.

IMO, income is income so it whould all be treated the same. Yes, the principal of the investment has already been taxed. However, the interest earned (capital gains) is what is being taxed - NOT THE PRINCIPAL. So it should be taxed the same as all other income.