Same problem with the Otterbox Defender 4S case. Just received a new one under their outstanding warranty replacement. The screen cover is flawless. I could open the control center less then 5% of the time via the "normal" way I always use my iphone. I have tried swiping from all different directions, moving my finger at different speeds & angles and soaking my hands in warm water to raise the temp in my hands. I can easily swipe or tap anywhere else on the screen. The control center "handle" seems too low to place my finger on it like I'm used to doing elsewhere on the screen. I can easily open the Notification Center on top.
This is what finally worked for me. I finally started trying to "swipe" the screen while first touching the diagonal part of the plastic case and then sliding my finger down on to the "glass." That works about 99% of the time for me now. It took a bit of practice to get the feel where to touch on the plastic diagonal part of the case, but now it's working much better for me. I can open the Control Center from almost any position along the bottom excluding where the camera icon is. I suspect it will also make a difference in the size of people's fingers on how well that will work.
I do not see it as a design flaw in Otterbox or other cases. They are doing what they were designed to do before the iOS 7 update came out.