Let's be clear here, the book wasn't too long. I just think that JK backed herself into a corner with the whole "yearlong" structure of the first 6 books, so she felt that she had to have Harry et al. spend their 7th year of school defeating Voldemort. Furthermore, by having Hogwarts being no longer a safe haven, she had no choice but to stage virtually the entire book away from Hogwarts.
Having done this, we're faced with a book that's about 90% Harry, Ron, and Hermione camping all over the English countryside (couldn't they go anywhere else?). No Neville. No Luna. No Fred and George. No McGonagall. No Snape. No Hagrid. No Tonks. No Hogwarts. Bleah.
It's axiomatic that the protagonist of any good work of fiction is generally someone fairly uninteresting. It's the antagonists that are interesting. It's the events s/he experiences and people s/he meets that make for a rollicking read. In the case of the Potter books, all 3 main characters fall into the "not very interesting" clique. Consequently, most of HPatDH is a fairly dull read. Large sections of the book are dedicated to nothing, or very little, going on. And lots of time needlessly passes.
In the end, though, JK redeems herself with a damned good finale. I'd've preferred half of the book be the final battle, or a series of battles, rather than the lackluster leadup, but then again, I'm not the billionaire novelist.
Three final thoughts:
1. So Harry and Hermione are by themselves, for months, in a single house. They're in mortal danger. They're terrified. They're teenagers. They're attractive. And nothing happens? Please. Even good, proper British kids would've been screwing in a week.
(here be spoilers)
2. After a little online reading, I've accepted that Neville could have been able to pull Gryffindor's Sword out of the Sorting Hat, but a bit more exposition would have been nice.
3. What's with the half-assed epilogue? How many kids really end up with their high school sweethearts? Isn't it a bit creepy to name both your kids after your dead parents? Doesn't Ginny get to name someone after Fred? Are we supposed to assume that Victoire is Fleur and Bill's daughter? What kind of name is Victoire? And what is Teddy still doing at Hogwarts at 19?