My biggest gripe with most fantasy books (and the occasional sci-fi if it involves alien worlds) is lack of depth.
The first series that ever impressed me with diversity was Dave Duncan's Magic Casement. Can't remember if that's book one or just one of the four. Doesn't matter. Duncan's world encompassed "races" that were all variations of human but with distinct physical and cultural characteristics, and those differences played into the story. Loved it (Duncan had a varied background himself in both education and employment).
TT: Anne McCaffrey actually did the same thing, but with so much subtlety I didn't realize what she'd done until years later. She remains a master of the craft, and one of my "mentors."
Another thing Duncan did to impress me was muddy up his stereotypes. He added gray to a world of black and white. Good guys could do bad things and vice versa. I couldn't tell at first glance who would play what part. Depth.
Katherine Coble recently said books written by those under 35 tend to lack flavour. While there are people half my age who've lived twice my life, she's not wrong. Experiencing life should translate to depth in writing.
I'm not saying longer books are better. Fred Warren is a master of conveying powerful emotion with few words. I am saying I wish more well-written books had a feeling of history to them. A sense that this world existed before you opened the book and will continue after the last page is turned.
Happy Wednesday, dear readers. May your world have only as much depth as you can handle with God's help.