Some of the negative feedback compels me to jump in with my generally optimistic perspective on this movie, the series best in my view.
I can’t help but notice just anecdotally that there seems to be a relationship between someone’s familiarity with the book and the degree of criticism they have for the movie. Some of the most enthusiastic praise I’ve heard is from those who either haven’t read the book, or read it a while back and don’t remember a lot of it. This isn’t new, it’s easy to spot all the omissions from a book and ponder how much better it played out in the format with a whole lot more room for elaborate setups and payoffs.
The thing is , I can understand why all of these most contentious choices were made in this one. It’s not that there’s not a little purist inside me, I’ve been critical of a lot of things in these movies. I think that they pretty much botched my favorite book in the series (Order), and all of the previous movies have near unforgivable moments for me (say the absences of any explanation of who the creators of the Mauraders Map were in PoA when it would have taken ten second in a scene that was already there at the end).
We mustn’t let the big picture slip away as we focus on the little things. Look at the scope of crafting a penultimate (sorta) chapter of a huge movie franchise. Think about the difficulty of capturing the general feeling of the magical world at war when you are adapting a book that is overflowing with detail and stray plot points, most of which the fans hold dear.
There’s a reason the critical consensus is that this is the best of the series from a purely film standpoint. Little things like the opening scene with the camera flashes from the mob press descending on the emotionally destroyed protagonist in slow motion, the transition to the otherwise normal boy flirting with a waitress while he reads about the weight of the world on his shoulders. This stuff wasn’t in the book, but those first minutes speak volumes even to the Potter novice. This is serious stuff.
As far the Burrow scene, the obvious main reason was that they needed another action scene to make up for all of the cut action from the finale (also a necessary move in order to make the final battle as awe inspiring as possible). Beyond that though, the scene also served to illustrate a few more things that we may understand as readers of the books, but needed a little more fleshing out for movies viewers. It shows that Hogwarts is indeed the safest place for Harry, the Death Eaters really do know no mercy, it adds to the tension and makes the viewer feel very uneasy about everyone’s safety outside of Dumbledore’s protection.In the book we had hundreds of pages of characters talking about how unsafe they felt and how dangerous this world is at this time…But in movies we have to show, not tell. Also, the house can be rebuilt.
Dumbledore asking Harry about his relationship with Hermione, once again we are blinded by our knowledge of the books. We all take for granted that Harry and Hermione are platonic, but in most movies where two characters come together in their heartbreak over their individual unrequited loves that’s a big flashing neon sign that says these two characters are going to get together. That one line of dialog just clears everything up for the viewer and assures them that the dynamic between the trio is not in fact going to turn into a love triangle.
This certainly was not the book to delved into Snape’s back story with James or Lily, nor should the movie be.
The finale was necessarily changed quite a bit. They really could not have had all the students and teachers and teachers join up to fight off the death eaters for obvious reasons. As for why the lead Death Eaters showed up…first of all they didn’t know what kind of resistance they would find, for all they knew there it could have played out the way it did in the book. Secondly, their job consisted of a lot of sitting around waiting for stuff to happen, these twisted people certainly wanted to get their hands dirty, and witnessing Dumbledore’s death would be both gratifying and confirmatory.
There were a lot of other criticisms I could respond to, but I don’t really want to write my own book about it. I’ll come back for more though if needed 
