Unfortunately, despite the best efforts of Howard Gardner, Robert Sternberg and others, the IQ test remains the singular test of individual cognitive ability. The mysterious entity that it measures - g, for general intelligence factor - is still seen as the dominant variable in determining the intellectual performance of our brain. . . . The first thing to say about g is that it's an incredibly robust statistical phenomenon. This means that the same person will get a similar score on an IQ test at the age of 12, 20, and 50. Furthermore, his score will correlate nicely with his academic performance, at least in certain subjects. For instance, a 2007 study by psychologists at the University of Edinburgh found that general intelligence accounted for 58.6% of the individual variance in math performance, 48% of the variance in english, and 18.1% of the variance in art and design. Of course, that still leaves a lot of variance unaccounted for, even in those academic subjects, like math, that are supposed to depend on the very mental skills measured by IQ tests. This helps explain why Lewis Terman, the inventor of the Stanford-Binet IQ test, eventually became frustrated with his measurement. Terman spent decades following a large sample of "gifted" students, searching for evidence that his test of intelligence was linked to real world success. While the most accomplished men did have slightly higher scores, Terman eventually concluded that other factors played an even more important role.Lehrer goes on to discuss "implicit learning," the intuitive side of knowledge -- for example, even when we can't remember the answer to something, we can have a "feeling" that we do know it. There are now crude tests that measure some aspects of implicit learning, and it seems to be an important variable in accomplishment.
I would point to something else: energy. Drive, focus, ambition, the ability to keep working on a problem until it is solved and then move onto another. One friend of mine once remarked, "I find that intellectual energy always wins out in the end."