Under current law, when you inherit an asset your basis in the asset is the higher of the fair market value at the time of death or the decedent's original basis. Almost always, the fair market value is higher.
Under the Obama proposal, when you inherit an asset your basis will simply be the decedent's original basis.
Example: Dad buys a house for $10,000. He dies and leaves it to you. The fair market value on the date of death is $100,000. You sell it for $120,000. Under current law, you have a capital gain of $20,000 (sales price of $120,000 less step up in basis of $100,000). Under the Obama plan, you have a capital gain of $110,000 (sales price of $120,000 less original basis of $10,000).