'Improvement' in art is a complex topic. But before judgements can be made about the presence of the quality or not in one work versus another, its definition needs to be explored. What's interesting is that prior I concieved it as being more subjective than other disciplines, say math for instance, but I don't think that's true anymore. Modern math has a great deal of specializations, and it's similarly ambiguous to draw comparisons between researchers in different branches as to their 'mathematical acuity' or something.
If there is a to trajectory of aesthetic evaluations, it must approach some value. My tentative hypothesis is that gesture drawing may a clear self-contained unit. I have reason to believe that there is a fundamental intution of aesthetic appeal in force, gesture. The gesture dances on the line between structural rigor and capturing this abstract quality. The two aspects balance off each other. The force seeks to supercede its boundaries dilineated by form, rigor, but without that solidy it dissipates.
I believe this is the fundamentals of technical achievement, and holds outside of visual art. Music that builds to a cresendo, motions of dance, the concept of a building potential, climax, and falling resolution in a narrative, stress this same dynamic -- form given to these abstract forces.
Gesture develops one of the facets at the disposal of the painter. I believe similar forces may be expressed through the dimensions of color and value.
What is perhaps integral is a layer of fiction that wraps over this foundation. The fiction is what gives a work its relatability, that ostensibly touches us emotionally. It produces higher level concepts; tenderness, sorrow, melancholy, joy...