There are many who believe that in order to be "in shape", you have to maintain a moderate heart rate for twenty to thirty minutes three times per week. When you think of this method of "working out" in terms of stress-induced changes in the body, it is easy to see how the low intensity, long duration cardio fails to live up to the expected results of its adherents. There are benefits, however, to low intensity efforts. In some instances, low intensity workouts can help those who are severely deconditioned or recovering from injuries develop a modicum of cardiovascular endurance and stamina on the way to recovery. That's about it. By no means will lack of intensity result in the physical results desired by the masses in general and CrossFitters in particular.
CrossFitters know and understand that intensity is "the independent variable most commonly associated with maximizing favorable adaptation to exercise". What this means is that working intensely to produce as much power output during each and every workout will unlock a neuroendocrine response in the body that forces adaption of the muscles to the stresses placed on them. This, in turn, releases growth hormones and endorphins, limits production of cortisol, stimulates sensory awareness, improves the vestibular system, increases bone density and muscle mass, reduces fat, and improves cardiovascular health in a manner that low intensity exercise never will. Because the stimuli placed on the body differs daily over a wide range of workout modalities and across a broad range of time (i.e. short, medium, and long duration workouts) domains under the CrossFit method, the body is forced to adapt constantly to different inputs. This is also why we often feel sore after a hard workout no matter how fit we've become.
Here's a link to a CBS News report on exercising harder. Click HERE. The "experts" in health and fitness are slow to come to the intensity-workout table that CrossFitters eat from day after day. If you're a CrossFitter, you understand this. The workouts leave you completely spent with nothing left for another effort. If you're not a CrossFitter, come check it out. You can't "get in shape" for CrossFit, you just have to show up and start. Don't be afraid to get hooked! CrossFit is for everyone, but not everyone's tough enough to do CrossFit. It all comes down to whether you can handle intensity.