Having lurked here for a while, I think that one of the reasons that some LWers have difficulty with exercise and diet is due to a crisis of optimization: given the wildly divergent claims of popular fitness gurus and the murky state of exercise science research, it's hard to know where to start.
I'd suggest that exercise and diet are areas in which experience, persistence, and self-experimentation are unusually important. I've been engaged in the serious practice of some form of physical exercise for virtually all of my adult life, and somewhat more casually in the observation of others doing the same. Some thoughts in the context of popular 'quick fix' programs:
Doing something consistently for a long period of time, working hard, and working harder as you get better are much more important than what you do. Most exercise programs published in books or magazines revolve around 2, 3, or 4 month plans. Even if you're devoting a lot of time to it, that is nothing compared to the physically active lifetime of the average person. It's barely enough time to become familiar with the program, let alone excel in it. It's OK to try different things to find something you like, but when you do, stick with it.
Individuals are highly variable in their adaptations to exercise and diet, but also highly adaptable. Over time, you'll learn the more unusual aspects of your physiology. Be extremely wary of thinking anything as a limitation or weakness until you have considerable experience with it (by considerable, I mean on the order of years). Learn to work around problems by trying small variations, one at a time.
Everything works for beginners. As your "training age" increases, it becomes more difficult to make improvements, and the importance of record keeping, analysis, and outside coaching becomes more important.
High-intensity programs, such as the ones that are currently in vogue, are quite effective in the 8-12 week window that comprises most popular programs. This is why they're popular. When training becomes consistent, over a period of years, intensity becomes a tool that needs to be used more specifically.
Fat loss is a matter of caloric deficit. You can achieve caloric deficit through increased activity or decreased caloric intake. Decreasing caloric intake is faster and more effective for most people living the average western lifestyle, but doing both is best. In the context of fat loss, diet plans are an enabler for caloric deficit. Plans that emphasize special food combinations often work well, but this is subject to individual variation. The most effective parts of these plans can be summarized as: 1) don't eat crap, and 2) don't eat a lot.
There are three components to a well rounded exercise plan: strength, conditioning, and mobility. Do all three, consistently and over a long period of time. You'll figure stuff out.
I've just read the Four-Hour Body by Timothy Ferriss. It seems on the face of it like ridiculously valuable material, if true - like what the completed version of Michael Vassar's proposed reboot of dietary science would look like at the finish point if dieting turned out to be more susceptible to Munchkinism than in my wildest dreams. Ferriss also talks the rationalist talk quite well in this book, much more so than in Four-Hour Workweek; he cites the experiments and occasionally says things like "I spent a lot of money on this and I expected it to work and it didn't work at all" or "I tried this and it seemed to work and I have no idea why it worked and I think it was probably a placebo effect."
Does the LessWrong hivemind have an opinion about 4HB? Has anyone tried it and found that it doesn't work, or that it does work, or that it works but not as well as Ferriss thinks it should work?