(Allegedly) Not nabbing Bin Laden BEFORE he committed the greatest, (meaning worst), act of terrorism in history.
Things that were Bush's fault:
Letting Bin Laden escape our grasp in Afganistan
Diverting our focus and invading a totally non-related country
Either lying or being criminally mis-informed about the motives for invading said country
Risking young American lives to do so
Rushing troops into said country without proper body armor or training
Ignoring the Geneva convention, creating the atmosphere for horrors like Abu-Ghraib
NEVER apologizing for afformentioned debacle, creating even more willing terrorists
Ect.
Ect.
Ect.
To continue in this fashion would be tedious and un-necessary. (I havent even covered his treatment of the environment or his willingness to mix religion with politics.)