Sorry for the unprofessional nature of this post--I'm not even taking the time to double check authors' names and such...
As I've read stories and books to my children millions of times over the last 3.5 years, I've begun to notice something about the illustrations in children's books and their "restful" qualities (or lack thereof). If I can make a gross overgeneralization here, it seems to me that older picture books are much more restful in tone, numbers of images per page, and subject matter than most newer picture books. Am I going to take the time to find examples of these illustrations online to include here? No. (If I had that kind of time, there would be more posts on here; instead of waiting for that time, I thought I'd just post the text version of my thoughts....).
Interestingly, my daughter (3.5 years old) really loves the older picture books and consistently gravitates towards them. The newer ones she enjoys, too, but doesn't seem to go back to them over and over and over. I'm sure text/storyline has something to do with it, but I also wonder if the pictures themselves are part of the charm. Consider the following titles, all of which have fairly monochromatic illustrations or, at most, colored images on only a few pages:
Make Way for Ducklings (McCloskey)
Blueberries for Sal (McCloskey)
Play with Me (? Can't remember off hand)
Curious George--old ones (Rey)
Good Night Moon (Brown)
Millions of Cats (Gag)
There are also older picture books which have color illustrations on every page, but the colors are somehow softer and less intrusive than the bright flourescents of today. In addition, not as many colors are used per page and there are fewer images overall per page--all of this adds to less visual clutter. Consider the following titles:
Peter Rabbit (and others)
older Golden Books, such as The Color Kittens, The Saggy-Baggy Elephant, The Poky Little Puppy
Frog and Toad books (and other Lobel books)
Little Bear Books
Where the Wild Things Are
I could go on and on with this list. I realize that the vivid illustrations we have today in picture books were not possible in the earlier books. Some of the newer picture books have done a good job (newer Curious George books have cute illustrations; DK books have great photographs and usually put a nice number on a page). But some of the newer picture books, perhaps in an effort to "stimulate" a child's imagination, are visually cluttered and tend to get put aside more quickly than those books which quietly worm their way into a child's heart and mind.
(I'll try to post a list of non-restful titles at some point--just wanted to throw this initial idea out there and see what the peanut gallery thinks.)