People act like being "world class" is some big mystery, but not in the 21st century. Extensive research has come to the conclusion that deliberate practice, over a long period of time, with the appropriate guidance of coaches and mentors is necessary and mostly sufficient to produce expertise in a field.
No, I don't think anyone can be Michael Phelps. In fact, it is clear that physical advantages go a long way in sports and athletic events. In almost everything else though, the right approach can produce mastery. The Polgar family is a living example of that w.r.t. chess. And most people are not looking to become world champions - they simply want to be successful in their endeavors. For that to happen, you must apply the same principles. To lose weight, I agree that saying "just diet and work out" is difficult, but most effective programs are either a result of intrinsic motivation or someone implementing a gradual program where you first cut out some sugar, then all soda, then cake, then you start eating one good meal a day, and so on and so forth.
In my view, this post marginalizes human willpower, which I consider one of the strongest forces in the world. Sure you won't be Michael Phelps, but you could say "No matter how hard you try, you will never become a Redwood," and I think the result is the same. Most people don't aspire to be world champion swimmers or extremely tall trees. They want to be successful in their endeavors, and for reasonably well off people in the Western world (which I think describes a large part of this board), there really isn't anything holding you back.
Ericsson et al. published a review of this field (expertise) in The Cambridge Handbook of Expertise and Expert Performance (http://www.amazon.com/Cambridge-Expertise-Performance-Handbo...). It's 900 pages but if you can't make it through a serious book then you sure as hell can't succeed in anything that requires real dedication.
But the truth is that innate differences matter not just in athletics, but in any endeavor. Even the famous Polgar sisters you mention both trailed men who had later starts and less formal early training for their entire careers. UK marathoner Paula Radcliffe's career is similar in many respects.
In general, the "purer" an endeavor is the more decisive innate differences are. By this I mean that innate differences will matter more for runners than for basketball players, more for speed skaters than hockey players, more for mathematicians than for hedge fund quants, etc...
But there is a great deal of evidence that innate differences create uncrossable chasms in skills far outside of athletics.
Motor Skills: http://news.bbc.co.uk/2/hi/health/1209186.stm
IQ & Cortical Thickening: http://www.nytimes.com/2006/03/30/science/30brain.html?_r=1
Facial recognition: http://www.wired.com/wiredscience/2010/01/face-recognition/
None of this means that willpower doesn't matter. Of course it does. However, at least accepting the reality that we don't all have the exact same potential at every pursuit can be beneficial. For one thing, it's signal not to heavily invest in a tournament-style career, such as modelling or concert violin playing, unless you have a clear aptitude. Even for more forgiving careers, where merely being competent is enough to make a good living, it's rational to choose a career that aligns with your natural talents. Why put X amount of effort to be in the 70th percentile of some field when the same amount of effort would make you 90th percentile in another comparably in-demand field? Most people are happier doing what they're good at.
One heart breaking example I witnessed personally was a family member. He wanted more than anything to be a classical musician. From the age of four, he was taught by the same teacher who trained a very successful NY Philharmonic pianist (opera coach). He practiced many hours a day all through his school years and earned both undergraduate and graduate degrees in clarinet performance. By any objective measure he was a very good clarinetist. But he wasn't good enough to make a real living at it. Unlike his mother, who had perfect pitch by the end of grade school, he never developed it. He saw more than one talented peer surpass him in despite his extreme work ethic. In the end, after putting over 25 years of his life and his passion into it, he finally gave up on his dream and became a programmer. Unlike his experiences in music, he very quickly rose to the top of development groups and it was he who was the one surpassing others who had been working at it longer and harder.
I often wonder what he could have done if he'd given up his initial dream after just 10 years instead of 25.