A UI should follow the same rules of etiquette as human conversation - if it would be rude for a human to interrupt you constantly when you were trying to do so

A UI should follow the same rules of etiquette as human conversation - if it would be rude for a human to interrupt you constantly when you were trying to do something, it's just as bad for your UI to do it. A good notification system needs to be context-aware, to realize that there's a difference between a device sitting idle on a desk or in your pocket, and a person playing a game, writing an email, or watching a video. Even within apps, there should be context. I may be fine with an unobtrusive notification appearing when I'm reading an email, for example - but if I've got the keyboard up and I'm typing, don't place a button right where I was about to press a key, or shift the location of UI elements while I'm interacting with them. The trouble with the iOS (and Mac & Windows, for that matter) approach to notifications is that they try to be "user friendly" when they should be "user subservient."