Greg Mankiw has an excellent article in the New York Times. I've summarized the article in my title, but here are two telling excerpts:
One point, however, cannot be disputed: Even if President Obama wins all
the tax increases on the rich that he is asking for, the long-term
fiscal picture will still look grim. Perhaps we can stabilize the
situation for a few years just by taxing the rich, but as greater
numbers of baby boomers retire and start collecting Social Security and
Medicare, more will need to be done.
Ultimately, unless we scale back entitlement programs far more than
anyone in Washington is now seriously considering, we will have no
choice but to increase taxes on a vast majority of Americans. This could
involve higher tax rates or an elimination of popular deductions. Or it
could mean an entirely new tax, such as a value-added tax or a carbon tax.