Improvisation

The article we read for this week's Q&A on improvisation provoked a lot of questions for me. I understand that improvisation can make music (or art, or public speaking, or teaching) better. In fact, I think it may be essential. I remember learning in Philosophy of Music that if a computer plays a song 100% perfectly, it is not pleasing to listen to as humans (I could be remembering this wrong so please correct me if I am mistaken). There is a human element that makes music pleasurable, and that element may be improvisation, even if it is not done consciously. However, I am wondering just how much improvisation is enough, and how much is too much. Deviating from the rules (both explicit and implicit) can make something more interesting, but deviating too much could be confusing and unpleasant. How much improvisation is too much? Is there a fine line or do you simply know it when you hear it?