Too Damn Small
It sure seems like pretty much everyone agrees on one thing Apple got wrong with iOS 5: the close buttons in Notification Center are way, way, way too small. I too misclick nearly every time.
A simple solution to the problem: they should have made the button 44 pixels wide (88 Retina display pixels) at a minimum. If it’s not that big in any dimension then it’s too small - the HIG tells you that.
That said, that panel is full of problems, particularly the Vista-like translucent bar that the button sits on. It’s just gaudy. I’ve actually gotten used to the linen, but that might have been a bit much too.