You can also see the same progression in the covers of Dragon Magazine over the years. The artwork for the first 5 years or so was often not the greatest technically, but there was much more variety than in later years when the magazine got slicker and fancier.
I think Tolkien's own artwork fits into this mold. It was very simple, but it captured a feel that is often lacking in more recent artwork, and let's not forget the immortal Hildebrandt brothers, who's style I always loved. Their character work was always amazing, but their architecture could be really bland. But ultimately it was a simple style that really captured the spirit of the books and the times in which they were done.