It's time to grapple with the new 3.8% tax on investment income. The ordeal of 2012 taxes is barely over. But it isn't too early to understand and cushion the blow of the investment-income levy, which Congress passed in 2010 to help fund the health-care overhaul. The tax, which took effect Jan. 1, applies to the "net investment income" of married joint filers who have more than $250,000 of income (or $200,000 for singles). Only investment income—such as dividends, interest and capital gains—above the thresholds is taxed. The rate is a flat 3.8% in addition to other taxes owed. "Affluent...