I'm partial to this description: (This is about martial arts, but I think it's applicable to fitness in general, or more broadly to any kind of increasing mastery.)
"Commonly, students' ability to see their errors and technical failings increases faster than their ability to correct them. Then the instructor faces the problem of discouraged students who believe they are actually getting worse through training rather than better....An analogy that may help the intermediate student is that of 'carving a cube into a sphere'. Training is the process of chopping off corners. Initially, the corners are lage and easy to see--as is progress. Later, each corner cut off reveals three new corners, albeit smaller ones. This process is endless, and while an advanced student may appear to others of lesser experience to be a perfect sphere, the individual is often painfully aware of the many corners that still need polishing." --Elmar T. Schmeisser, "The University Dojo" in _Martial Arts Teachers on Teaching_, Carol A. Wiley, ed.