And what constitutes a notification? That's the big problem and this is why they probably didn't release a notification center. To run more usability tests to see how the people use their windows phones. An email is a notification but when? When received in the phone app or in a third party email app? A sms is a notification but how about my whatsapp im message is it also a notification? Microsoft apps mostly never did stuff that 3rd party apps could not do(desktop side or Modern ui stuff: wp8 w8). And if they would do they would surly be bashed.
So making a light blink on all notifications will only invite spam. Look at all the android games and apps that spam your notification center. And opting out notifications is something that no one is sure that is easy enough and usable enough for new users. So you could get this selected behaiviour for only select oem apps(phone, mail, sms) and this will create a storm
The notification center problem will be sorted by the microsoft and I'm sure it will be a beautiful solution. Look what they did with the gestures in windows 8 and how much usability tests they made just to understand how people use a product so they can make a good user experience.