A Mensa president once said that a student's success in elementary school can often be attributed to intelligent, but from high school, college and and later in life, it's unpredictable as it has much more to do with their willingness to work than the plain intelligence.
Also, what JohnWillkins said: "nature or nurture"?
I watched a documentary that argued over it, and it didn't come up with a clear conclusion, but just let the viewers conclude on their own. What I understood from that documentary is that it's very important to nurture "willingness to study" from when kids are born. I think it was back in 60's, they did an experiment with underprivileged kids from poor suburbs. I think the statistics showed that many kids who grew up there ended up in jail, sold drugs, died young etc etc and basically didn't even finish elementary school.
What scientists did was that they chose several underprivileged kids (and I guess most of them either had single-parent household, and even that parent didn't have much time to take care of them as they had to work a lot) from that neighborhood whom they'd bring on tests and monitor their growth over the course of their lives. They regularly brought them to their center where the kids would play with mentally stimulating games every since they were born (starting with just placing a cube in square opening in wood and eventually passing on to IQ tests) and watched their progress.
The results showed that the kids who were regularly mentally stimulated succeeded in their lives, regularly finished high schools and most of them finished colleges. It was a big contrast with the other children who had the same household conditions but were not part of the testing.
They said it's because they managed to teach them that learning is good.
So I guess what I am trying to say is that intelligence by itself certainly helps in many situations, but it is not enough. It has to be fostered and nurtured regularly in order for anyone to achieve their full potential. And that involves hard work! =)
But if we talk about success, I think the most important ingredients are intelligence, willingness to work hard, passion and of course, luck