For the Newbery Medal for children's literature, that means contemporary and historical fiction about Overcoming Hardships, societal or familial, almost always prevail over humor, adventure, and fantasy or science fiction. Stories that bring us into Other Cultures also have an edge, as long as those cultures are real. Of course, there are exceptions, like 2004's Tale of Desperaux, but the pattern holds up generally over several decades of winners. It's just the way our culture tends to think.
I looked back over the list of Newbery Award books (just the winners, not the honor books), and I found:
54 fairly serious books of contemporary and historical fiction about overcoming hardships.
19 books set in foreign countries, other than Britain.
14 books that could, very loosely, be classified as somewhat humorous, although most of those still dealt with serious themes, too. Example: Caddie Woodlawn is historical fiction about a girl growing up in a frontier town. It has humor, but it's also about being brave and facing the hardships of frontier life.
12 sort of fantasy-ish/science fiction books. Almost all of the fantasy and science fiction books were, again, serious books with grand philosophical themes. Except for The Voyages of Doctor Doolittle and maybe The Tale of Despereaux, the latter of which I haven't read yet.
Pure adventure? Roller Skates? The Westing Game?
So, are the books that really last, the classics, really all serious? I can name some classics that are certainly not serious: The Three Musketeers, all of Wodehouse's books, Anne of Green Gables, The Adventures of Tom Sawyer. Are these the exceptions that prove the rule?
Rule: Award-winning books must be Serious Books that deal with a Serious Theme ---mostly.