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.