Some surprising titles that I'd not considered to be MG (but Amazon does):
Percy Jackson series
Harry Potter (1 through 7)
His Dark Materials trilogy
The Chronicles of Narnia
Septimus Heap books
Diary of a Young Girl, by Anne Frank
City of Ember
A Wrinkle in Time
The Earthsea books
...um. Wow. I've read most of these books*. I used to emulate these books**. Clearly, I have had a skewed idea of MG. And I think you guys are right...there is a distinction between upper and lower MG. Heck, there's a distinction between Harry Potter 1 and Harry Potter 7. But in Amazon's 9-12 category, there are books as varied as Shel Silverstein and Dr. Seuss to Christopher Paolini and Louis Sachar.
Thing is, I did not consider any of these books to be MG (with the possible exception of Narnia and early HP). I just....didn't. A Wrinkle in Time? That was the epitome of YA to me...but it says, right there, for "ages 9-12." I thought the Earthsea books were adult. Stargirl is set in high school. Anne Frank's diary is about the Holocaust. I thought The Giver was way above MG.
True, there are some books on Amazon's 9-12 list that are labeled as YA if you click on the book for more details. (Eldest was the first I found.) And Percy Jackson is under the children's and the teen section.
So...um...all I've learned is that I need to do some more research, and that MG is definitely not what I thought it was before.
* And the ones I haven't were on my to be read pile. Except for Pullman's books cuz, yanno, Pullman sucks.
**Again, except for Pullman. See previous footnote.