Usual spoiler alert, yadda yadda. It doesn’t spoil the seventh book, but it infers directly to plot elements and a few specific passages.

http://www.ew.com/ew/article/0,,20044270_20044274_20050689,00.html

Bottom line:

Rowling has been far more successful, critically as well as financially, because the Potter books grew as they went along. That, I think, is their great secret (and not so secret at that; to understand the point visually, buy a ticket to Order of the Phoenix and check out former cutie Ron Weasley towering over Harry and Hermione). R.L. Stine’s kids are kids forever, and the kids who enjoyed their adventures grew out of them, as inevitably as they outgrew their childhood Nikes. Jo Rowling’s kids grew up...and the audience grew up with them.