There are two parts to the textbook adaptation equation: stimulus and response. There's one factor missing.
Absent is Environment. The effect of anything new to you is affected – enhanced, dampened, even dismissed – by your worldview. Consider trying to sell Obama's health care plan to a fiscal Conservative, or cutting back government services to a Communist; they're not going to go for it, ever, because their frame of reference is different.
When we do CAT Testing on new clients, there's typically one glaringly obvious weakness that shines through in each. Obviously, they're inefficient at that particular fitness trait, and it's because they've adapted least to that particular stimulus.
Take this example: a human male with a lot of strength training background scores low on the Aerobic test. He's least efficient at aerobic metabolism. This means if we expose him to equal part cardiovascular activity, strength training, and METCON work, he'll adapt first to the cardio…because that's where he's least efficient.
Weight trainees (predominantly men) who have gained a little muscle under the bar become nervous about doing much aerobic activity, for fear they'll lose some muscle mass. So they walk, very slowly, on treadmills to do "cardio." Or they'll do nothing. Unfortunately, this is a vicious cycle, and it's perpetuated by bodybuilding magazines who make their dough on supplement sales (Men's Health, Men's Fitness, FLEX, and the others: all owned by supplement companies, if you trace back far enough.) Soon, guys who "work out" are scared to do ANY aerobic activity for fear of looking 'flat.'
When I started CrossFit, I quickly dropped 20lbs. It was VERY hard to adjust, mentally, to weighing under 190lbs after years of competing at Powerlifting. No, I wasn't fit; I was just strong and a bit bigger than I'd ever been. Now that I'm a more efficient runner (thanks Mike!) I'm able to gain weight without worrying about getting slower. I've adapted to the aerobic exercise.
When women start CrossFit, we advise them to lock their scales away for two months. Typically, they're starting with little muscle mass, and they'll gain muscle and lose fat at close to the same rate. They take awhile to adapt to strength training……but their clothes feel looser, they're much stronger and faster and fitter and closer to doing amazing things.
When you try anything new, you'll have an easier time if you're more aware of your current state (the environment of Your Self.) For instance, you're usually either gaining or losing weight, not staying the same, and it takes time to halt that process first before the train can start rolling in the other direction. Critics of CrossFit ("I got too skinny!") are usually those who have dropped out after the first month; they've exposed their weaknesses, but don't allow time for adaptation.
The beauty of CrossFit is that adaptation in one area doesn't require regression in another. It's NOT true that the act of rising to a "9 out of 10" as a runner means that you fall to a "2 out of 10" as a weightlifter. Not at all. These traits are NOT mutually exclusive until you reach a very elite level. Being a generalist – specializing at NOT specializing – does not mean that your target is a 5 out of 10 in all areas; it means forcing a REAL adaptation.
I'll leave you with this question: does rapid fat loss, as you'd see from a Rice Krispies (now with whole grains!,) The Biggest Loser, the South Beach Diet….force a long-term adaptation, or just a short period of physical stress? And if the latter, what do you think the adaptation will be to that stress in the long term?