Old playwriting thing, which can also be applied to comics (and of course, books): If you're going to put a gun in the drawer in Act I, you'd better use it by Act III. Conversely, if you're going to use a gun in Act II, you'd better put it in the drawer in Act I.
Rowling used a whole lot of stuff here that was not in the drawer in Act I, so it comes off feeling like a bit of a cheat. I understand that when she was writing Book One, she didn't even know if there'd be a Book Two,let alone a Book Seven, so we can't exactly expect a Master Plan... but by the time she was writing Book Four, she had to know that there was going to be a Book Seven. She had plenty of time to sow seeds, and she didn't do it.
So yeah. My reaction to the book is a bit meh. It could have benefited from some tightening of focus, some editing. She could have pulled it all together using pre-existing elements. But she didn't. So. Yeah. Meh.
I haven't read anyone else's reactions, so this may make me wildly unpopular, I don't know. Could she have done better? I think so. But could anyone have really pulled it off to everyone's satisfaction, this unprecedented juggernaut it's become? Probably not. Not without hiding the