For his incompetence.

For forcing the Taliban out of power in Afghanistan (a very good thing), but then letting them regain power by moving troops and money from Afghanistan to his ill-considered war in Iraq.

For squandering all the good will that American had been building up in the decade before he took office.

( Detailed Rant Ahead )

Most of all, he'll be remembered for his blind arrogance that led the world to the brink of yet another world war.