Dubya has made similar and worse gaffs. Not just worse than Kerry's goof. Certainly worse than any recent resident of the White House, and quite possibly the worst in history. I think we'll actually know in a few years when search engines reach the level of sophistication to handle semantic queries of the kind 'scan all presidential gaffs and rank them by magnitude and consequences'. Even with over 200 years of presidential statements to search, I'm sure Dubya will dominate the bad speaking categories. I hope the records include some of his beloved fart jokes, so he can compete with Nixon in the profanity categories.
The Democrats are simply not able to get any traction on Dubya's goofs. It is simply 'understood' that Dubya says stupid things. Dubya's statements can never be taken at face value, but are always pending clarification or dismissal. Dubya screwed up again? No news here, move along.
The detailed notes describing Kerry's intended speech were released BEFORE he made the gaff. We do know exactly what he meant to say, but no one bothered to check? Yeah, it wasn't a very funny joke even in the original form--but there is an easily checkable fact there. They didn't bother or they didn't want to bother. They already had their sound byte in the can, and who cares about the facts.
The difference from Dubya's errors is simply in the MSM handling of Kerry's gaff, though I think the thing with Dean's scream was an even better example. I actually think Dean was probably the best candidate the Democrat's had in 2004--but he vented on film and the MSM was able to toss the nomination to Kerry, who was clearly not the best of all possible candidates.
Truth be known, Dubya's mistakes are not particularly interesting. Most of his officially approved public statements are also of no interest, since he's just reading what he's been told to say. The only slightly interesting thing there is how much trouble he has reading it, even with preparation and given that they are trying to write for him, with good knowledge of his linguistic problems. No, I think there are only two interesting aspects of Dubya's statements.
One interesting aspect is whether or not he knows when he's lying. At this point I am quite convinced that some of his lies are deliberate and with full knowledge that he is lying. (This is in contrast to Reagan, who I have come to believe simply saw everything as a script that defined the reality of that moment.) As a resident of the White House, Dubya is simply an amazing combination of ignorance (resulting in many counterfactual lies) and outright stupidity (resulting in many self-contradictions, mispronunciations, and other obvious errors). Dubya doesn't even notice--or care. (Rove cares--but only because the loss of credibility damages the marketing of the product.)
The other interesting aspect is when Dubya slips out an embarrassing truth. The specific example I regard as most offensive was when Dubya was filmed pumping his fist and saying he felt good just before he made the public announcement of initiating a war. This is the WRONG state of mind for someone who is initiating violence that is going to kill human beings, but the hideous truth is that Dubya felt good about announcing that he was causing people to die. Even if the rationale hadn't been a pack of lies, Dubya still should not have felt good about the necessity for violence. There are times when force is the only option, but it should never be a happy option. A 'good person' cannot feel 'good' about starting a war.
In conclusion, Dubya is the comma the historians will dismiss. From a historical perspective, I predict that there will be two primary historical foci for this period. One will be how Cheney's Coup captured the government for the neo-cons and big business, and the other will be how Rove created the neo-GOP on the carcass of the old Republican Party.
(I think I need to mention Dan Quayle in passing. Yes, he said some things that were even more stupid than Dubya's statements--but we were simply lucky enough that he never actually became the resident of the White House. However, it is an additional symptom of the problem that many of Quayle's gaffs are now falsely attributed to Dubya--because they're "close enough for government work.")