Part of the charm of the books is how nonchalantly they shrug off horrible events--when Harry loses all the bones in his arm they just sigh and give him a potion and that's the end of that. On the other hand, one of the most mind-numbingly frustrating things about the books is that they take the same attitude with legitimate danger. The end of the fourth book is essentially the first time the adults get in gear and realize that this is bigger than prep school shenanigans, and the world breathed a collective sigh of relief. I think that if Rowling goes back to her standard formula, people will hate her.
She's really at a turning point--the end of the fourth book promised a lot of more serious adventure than we've ever seen before, and on top of that her core audience is getting older. But at the same time, her formula has worked very well so far and she's probably reticent to leave it behind. If this book is just another romp at school where they get into detention and ignore the serious dangers, there will be a big backlash. If the book strays too far from the formula there will be a similar backlash. She'll be treading a very fine line, and I'm interested to see how she pulls it off.