Yeah, I agree, the MCAT is more nerve wracking then the USMLE. Some 95% of US med students who take the USMLE pass and become doctors, whereas only 40-50% of students who take the MCAT's get into med school. I remember the MCAT being more difficult to study for too, because it didn't just test regurgitation of facts like the USMLE often does.
Yup, the mysterious "g factor," or general intelligence factor, that psychologists like to talk seems to be more dominant on a test like the MCAT than the USMLE. But even for the USMLE times are changing, and being able to think on your feet means more than it used to for this test.
If you look back to the released Step I questions from god knows when (maybe late 80's, early 90's), there were lots of simple recall questions requiring little to no thinking, if you could pull the knowledge out of your head. There were also more than twice as many questions and it was a two day test.
Now, fast forward to 2004 (see 2003 released Step I questions). Only half as many questions, but there aren't as many simple recalls. A larger proportion of the questions require conceptual thinking and application of knowledge: multistep problems, strange vignettes, and the occasional weird research protocol (and least to the biochemically ******ed). The g factor means more when the test becomes more conceptual and less regurg. There are still plenty of recall questions (in the guise of vignettes), but it seems like you have to go a step or two further in the chain of thought than you used to.