Richard Clarke has a partial list in the Washington Post article linked above. He mentions global warming, the continued existence of al Quaeda, the growing power and pugnacity of Russia, the spread of nuclear weapons, war in Africa, and contining problems with Pakistan.
He doesn't mention the increasingly unstable position of the US economy, or the serious challenges to our internatinal competiveness, but it hardly matters. Bush remains the most stubborn and foolish of stubborn fools, completely committed to his follies. A Democratically controlled Congress can do some damage control, if Republicans in Congress cooperate, but probably not a lot.
Whoever wins the White House in 2008 seems certain to inherit a greatly diminished country with a boatload of terrible problems.
Meanwhile, Europe is more threatened and seemingly even less alert to the danger. Putin has Europe by the throat with his control of gas and Russian oil, and Putin is known to consider the roll-back of the Soviet empire as "the greatest tragedy of the twentieth century." If (or when) he decides to re-annex the Baltics or a bit more of Eastern Europe, who will there be to resist? The mini-countries involved? The European Union which is largely unarmed? Bush has largely exhausted the American military.