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.
That said, the newer sets do still produce art with that charm from time to time. But it's much rarer than it used to be.