Principle of Least Surprise

I will point you to a link on Wiki.
Basically, I think it is inherently tied to the user interfaces for various software systems and applications. As a Mac user at home, I have this habit of trying to close applications with an Alt+Q at work. This is before I realize what I am doing and then switch back to Alt+X.

But I think this principle goes further than that. Basically, it is binding the user interface for all new systems/applications to the existing conventions. This is probably why you see most operating systems run some variety of windows (though they may like to call them something else --- the concept is the same: you open a new window and then close it.) Very few new applications come up with newer paradigms .

So, the question is: is this a good principle to follow while designing user interfaces ? How do we balance convention/familiarity with giving users a vastly new experience, a paradigm shift ? What is the optimal solution ?