I think kickboxing may sometimes feel less intense, without necessarily being so, because the effort is split between upper and lower body. More muscles are working, so even if the individual muscles are not working as hard as they would be with a step workout, for example, the net intensity to the body can be the same or more. (The same is true for functional weight workouts: though one uses lighter weights, and the moves might not seem as intense, because they are spread out among many more muscles, they can be as intense for the body---and are said to burn more calories because of that).
That being said, I think that another reason for KB not being as intense for some may be form. Punching with just the arms uses much less intensity than using the whole body to punch with (starting with the push-off from the floor, with the energy going through the legs, into the hips, through the core and into the upper body). Also, it's very easy to punch and kick with the correct form, but a lack of power. And that power, in my experience, comes mainly in the recoil. Try to get the punch/kick back in as fast as you can (also safer on the joints than concentrating on sending the punch out, which can lead to locking the joint and joint trauma). Imagine punching into a flame---and you don't want to get burned. Or think of your hand as a snake's or frog's tongue, going out and in fast.
And make sure to do full punches (extend to about 95 degrees, and make sure hands come all the way in before the next punch) and kicks (chamber both on the out and in moves).
I noticed that in her earlier KB workouts, like CK and the KB on CTX, Cathe seems to find some of the upper body moves very intense, and even has us put our arms down at times to give the upper body a break. I assumed this was because it was a type of workout that was fairly new to her, and when your body hasn't adapted to a certain way of using it, it's tougher.
