Yglesias makes a sharp point here:

One possible explanation for [Bush's failures] would be that he’s dumb. Or alternatively that he’s incredibly lazy. The president of the United States has a tough job, after all, and it’s totally possible to imagine a person with roughly correct ideas to nonetheless screw it up through incompetence and blundering. But if conservatives want us to believe that the United States blundered through a major terrorist attack, two major failed military adventures, dismal economic performance, and then finally an epic economic collpase all while under the watch of a very bright and attentive leader then it seems like a much deeper failure of the movement.

Intelligence and ideology were major causes of Bush’s disastrous reign, though there were many others.

I actually think it’s possible to make the scope argument here: that the presidency has grown so ridiculously large and unwieldy that it’s impossible not to find several areas to specialize in and pay only cursory attention to the rest. I definitely think you see this with the Obama Administration, where some areas of public policy are just completely delegated because there’s too much to do. Problem with that is much of that growth has been due to shifts in the scope of foreign policy since WWII, and Bush was absolutely horrible at managing foreign policy. Apart from the wars that sapped any post-9/11 goodwill in the Arab world, Bush managed to tank relations with Russia and much of Europe (with the exclusion, of course, of Poland), allowed Hamas to gain official power in Gaza, and so on. Bush had little foreign policy preparation before entering office–substantially less even than Barack Obama, with those couple years on the Senate Foreign Relations Committee–and while he seemingly spent more than the appropriate amount of time on that subject for a president, to the detriment of domestic policy. What with the floods and bridge collapses and all that.

I think the other factor, aside from intelligence and ideology, that led to Bush’s disastrous tenure was character. Bush was no judge of it in others, and his flaws were so apparent that it made it easy for tougher, cannier men in the White House to handle him and turn him to their views. I’ve written in the past that you can make the argument that Bush grew a little bit during his time in office, and that he eventually learned his lesson and stopped letting Rove and Cheney and Rumsfeld jerk him around so much. But ultimately, a president who hadn’t been born to George and Barbara Bush would never have been able to rise to the top without finding ways to keep those flaws in check. Dubya basically went from nothing to a weak governor job to president, and never had to confront his own failures and make changes. There’s no real substitute for it in growing personally and careerwise. This is, perhaps, why the Bush years were so frustrating, because the guy had never really had to grow up.