I think a lot of what the MCAT tests is your ability/willingness to study, work hard and prepare for an exam. If you step back and look at what it covers (first year of Bio, Chem, Physics and Organic), most pre-meds have at least seen almost everything that can be covered on the MCAT by the end of their sophomore year. Granted, most of us probably sucked at a lot of it (especially organic), but at least we'd seen it before. So the concepts are not mind-blowingly hard, but the test is just so incredibly broad and it covers a SOO MANY different semi-dificult topics, that almost nobody can just simply remember every concept and every formula from their first two years of college. Almost no matter how smart you are, you will not do well if you don't prepare. So I think most of what it tests is your ability to focus and prepare yourself, as opposed to just testing your raw intelligence.
I would say people who score well on the MCAT tend to score well on the USMLE Step 1 for two reasons;
1) They're somewhat bright (~20%),
2) They study for it and work very, very hard (or very effectively) (~80%)
Those that worked really hard and studied a lot as a pre-med will probably continue to study and work really hard while in med school. The effort that got them a decent MCAT score will get them a decent Step 1 score. But there is definitely a portion of it that just represents intangible ability to just pound/beat/bludgeon the right answer out of a passage or list of possible answers.
This is a completely off-the-cuff, totally unsubstantiated opinion though... not unlike many things on SDN.
