No Imagination, no inspiration = bad art.
Adventure art has to be imaginative!
It must inspire the viewer, It must hold your attention and fire your imagination.
If it does not do these things then (despite its technical quality) it is POOR art.
While 3E art has shown an increase in technical quality, The core rule books have shown poor imagination in thier artwork (with the MM3e being a little bit of an exception to this, but still not meeting the grade).
1Ed art was admittedly less advanced in its technical ability, but showed fantastic imagination.
Lets face it. Even if an artist can draw or paint a man with a sword with photorealistic quality, unless it contains a subject of interest and can hold the imagination of the viewer, then it has no more value than a decorative scrollwork around the page borders.
It seems that over time, role playing art as a whole has become more like draft work or technical illustrations than true art.
WOC have proven that they have access to quality art by the work found on the MAGIC(TM) cards that they produce. So the only thing that I can conclude is that the job of illustration of the core books went not to the top talent, but to the lowest bidder.
One must not forget that fantasy artwork was as much an influence on the birth of D&D as the literary works of Howard, Tolkien or Moorcock. If RPG companies would remember that, they would put a lot more effort into art than they have.
Here is a prime example of an artist who helped form the D&D model and who should be the benchmark for role playing art quality.
I present, the grandmaster of Fantasy art himself :
Frank Frazetta
http://www.wadhome.org/frazetta/