How should a bicycle fit? What are the characteristic features of a bicycle well-fitted to its rider? 

Are there a standard set of "rider positions" that we talk about in bicyle ergonomics? 

What are the standard tolerances and variances of these heuristics in practice?

How do these features vary by domain (eg. road, mountain, grocery-getter)? 

How do these features vary by application (eg. racing, touring, recreation, practical)?



Related (but not duplicate): 


How to do a bike fit? 
(This is similar but different because it's about doing a bike fit, versus what a good bike fit is.)
How should a bicycle helmet fit?
How is bicycle fit affected by switching from a riser bar to a flat bar?
Bike fit: Adjusting stem angle vs. height
To distil an entire field down to a short answer....


  The bicycle should be comfortable to the rider and for the rider's intended use.


How you go about getting to that state of comfort is a slightly longer-answer, and too big for this space.