In class, I used the example of the old white dude with a tweed jacket with suede elbow pads as a prototypical example of a professor. One of my students made the apt point that this sounded more like a stereotype. After pondering this, well this student was correct. This would be an example of a stereotype. Stereotypes may inform our prototypes, however prototypes are based on personal preferences (thanks to Sam for putting it this way) and experiences. So for example, your example of the perfect instructor results from your personal experiences of past instructors, your learning style, and etc, but your notion of the perfect professor may still be informed by cultural icons such as Robin Williams' character in Dead Poet's Society (is that cultural reference too outdated?).
(also, thanks to Suzanne for pointing this out in the first place!)