Because it's political season, lists of Bushisms like this one seem to be in vogue now. (I found the link to it on the front page of digg.com.)

Mark Liberman at Language Log has been at pains to point out how Bush, because of the stereotype that he is a mediocre intelligence and perhaps suffers some cognitive impairment from all those years of hard partying before he got sober, gets a bad rap for things that escape attention when they come out of the mouths of other people in public life.

Liberman makes several good points:

1. Even people with reputations as excellent public speakers can stumble badly, having moments of disfluency that look quite similar to many of the Bushisms that are commonly cited.
2. Bush has many moments of perfect fluency, and so cherry-picking the screw-ups unfairly portrays him as bungling far more often than is really the case.
3. The cottage Bushism industry, needing new material to feed on, trots out statements that are innocuous as if they are somehow problematic. (If you have to come up with a regular gaffe for a column, Bush may not always oblige you, you.)

I agree with all the above points, and yet I still think Liberman still lets Bush off the hook too readily. The thing about the disfluencies in Clinton, Kerry, and others that Liberman cites is that they just aren't that funny. Take a look at the top two examples in the Bushism list:

1. "I am here to make an announcement that this Thursday, ticket counters and airplanes will fly out of Ronald Reagan Airport."—Washington, D.C., Oct. 3, 2001

2. "Too many good docs are getting out of the business. Too many OB-GYNs aren't able to practice their love with women all across this country."—Poplar Bluff, Mo., Sept. 6, 2004

In both cases, these are probably the result of Bush getting himself boxed into a grammatical corner halfway through the sentence. With item number 1, he clearly meant to say something like "ticket counters will reopen and flights will resume". But having uttered the compound "ticket counters and airplanes", he was stuck without a good verb that would work for both, and so wound up with the ridiculous zeugma.

And I think Liberman's analysis of the second one is right on track. He points out that listening to a recording of the utterance shows that Bush hesitates at their, repeating that word, which strongly suggests he was groping for a suitable noun.

But understanding what happened to Bush in a performative sense doesn't really explain everything. At least for #2, Bush was almost certainly aware that he was in linguistic trouble. Why, then, couldn't he find some way to get out of the trap without embarrassing himself? It may be the case that Bush stumbles no more or less often than other public figures, but when he stumbles, he seems to produce unintended comedy more often.

The impression it gives is of a mind that is unable to make connections, unable to see the broader consequences of what is being said while it is being said. Consider, for example, number 6 on the list:

"See, free nations are peaceful nations. Free nations don't attack each other. Free nations don't develop weapons of mass destruction."—Milwaukee, Wis., Oct. 3, 2003

This quotation isn't disfluent at all. Linguistically, it's perfectly well formed, and the core idea, that free societies don't tend to war against one another, is a familiar talking point. What makes it funny is that his attempt to tie that trope in with WMD leads to the necessary inference, if you take his statement literally, that America is not a free nation. After all, it did develop atomic weapons. We know, of course, that Bush didn't really mean to imply that, but why couldn't he make the obvious connection? It's not exactly a subtle logical point. Bush clearly wouldn't want to be seen as a blatant hypocrite. The only tenable conclusion is that in the moment of speaking, Bush can't think beyond the words he is uttering to the larger implications.

That may be a failing of many people, but those people, by and large, don't become president. We have the right, I think, to expect a president with an IQ above the temperature of lukewarm spit, and the right to mock him when his utterances demonstrate that he lacks the requisite mental apparatus for the job.