I had a few issues with it.
1. Yeah, the part where they just kind of hang around in the middle is horribly long and rather dull. Not much going on, in regards to the info/words ratio.
God, that bothered me. Especially the lack of action going on during that time specifically - Harry and anyone associated with him (including Mr. Weasley, who was actually still working at the Death Eater controlled Ministry of Magic even though his family had known ties to a certain Mr. Potter) should have been the targets of a massive manhunt.
3. Why mess around with Dumbledore's past? Is there any real reason to not have a character that is actually good and not haunted by some terrible mistake they made? It's rather politically correct, in that you aren't allowed to have a pure good person or something.
Dumbledore was one of the least-touched upon characters in the book. Basically, all we knew about him was that he saw "woolen socks" in the Mirror of Erised, which we now know to be a lie.
6. Voldemort acted poorly, from a tactical point of view. First of all, he knows for sure all but 1 of his Horcruxes are gone. Does he retreat, biuld up again, make more? No, he decides to personally fight, putting himself at maximum risk for little to no explanation. Not only that, but he does not sufficiently abuse his strength: he only uses his "really loud voice" power twice, he wastes an opportunity to inflict more structural damage on Hogwarts, he decides to attack the enemy stronghold when they are at maximum power and morale, ugh... not a very good commander.
Voldemort's strengths lie not in strategy, but in recruitment and public relations. It's kind of strange to say this, but his real power came from his wide base of followers. Also - yes, he was foolish. But that stems from pride. He thought he was invincible.
He did not want to inflict that much structural damage upon Hogwarts because he planned on winning the castle for himself. He had good memories there, most of his childhood at least.
9. Plenty of unexplained stuff. How did Umbridge get Moody's eye? etc
She's a damn Death Eater. She must have been there during the attack.