Romney wants to eliminate the capital gains tax for people making under $200,000.

[Train wreck]

Capital gains income is generated from 4 main sources,

Buying/selling of Stocks and bonds,  buying and selling of precious stuff (like metals, art, coins, collectables) and land/real estate.  Most people who make under $200,000 do not have those types of assets laying around, so eliminiating the capital gains tax for those income groups will have little material difference.