Again, this was my favorite book, so it's hard to step back and look at it as a film apart from the movie. But as a movie alone, it was done fairly well. I enjoyed the plot, I thought they did a fairly good job of covering what needed to be covered and leaving out what could be left out. There was some superb acting, as I have already noted, especially from Luna Lovegood and Bellatrix Lestrange. The scene in the Ministry at the end was well done, and I was especially pleased with the way they managed to take bits and pieces of the long conversation that Harry has with Dumbledore in the end and weave them into the fight scene at the end of the movie. It meant you didn't have a long, boring conversation as the end of a movie, and you still got the important parts. I also didn't mind the lack of Quidditch; I think it was a very smart move to cut it to make more time for other things.
What I think they could have done better, though, was to cut a few more things in order to give some more actors more screen time for character development. I was very disappointed with Hermione and Sirius especially, I have always felt that they did fine jobs in the past, and Gary Oldman is a fantastic actor, and I felt their performances were stunted in this film. And after thinking about it, it seems to me that it's because they had very little time to show much character. Sirius had quite a bit more screen time than Hermione, but most of his screen time was devoted to furthering the plot, and not to his character. We really didn't see much of the frustrated, lonely, reckless Sirius that JKR wrote about in the book. We just saw someone who was, frankly, one-dimensional. And I think that is what is to blame for Gary's poor performance. He had little to work with. Same for Hermione. I have never thought that she had any problems acting before, but in this movie she fell totally flat, and I think that probably has to do with the 5 whole minutes she was on screen.
I think they could have given those actors more time, too, by cutting a few other unnecessary things. First, Kreacher. In the book he plays a very important role, but he was not relevant at all to the movie. Granted, the parts where he was on screen were short, but they still could have been cut. Even in the next movie, he isn't necessary, as Harry could easily just have Dobby follow Malfoy. Also, Grawp could have been cut as well. In fact, the whole scene with Hagrid was useless, if you ask me. Unless Grawp plays a big part in the last book, he is also unnecessary. And the way they changed that scene I didn't care for. They make Grawp this funny big kid, as though he's about five years old, when really he was frightening and Hermione was terrified of him. Again, that was a part of where I lost more of the character of Hermione. The kids could have easily just led Umbridge to the centaurs and then escaped without Grawp's help. And I wish that Hagrid hadn't just told them why he was gone, as it was a secret. That would have been another chance for Hermione to be more Hermione-like, at least. I do think the movie would have not have been worse without those few parts, and perhaps more time could have been given to some character development.
As I said already, the woman playing Dolores did extremely well. It was hard when I first saw pictures of her, because she doesn't look like Umbridge, who has a very distinct appearance as described in the book. But when I saw a few movies that actress has been in, I could see that she would be able to get the personality down, and she did. Wonderfully. Her clothing was hideous, and I like pink, so that says a lot.
I wish they could have done a little more with Fred and George, but the fact that they kept some of that in there was good. And Flitwick was funny. That was another thing that was missing, some of the teachers had very little screen time.
I still don't care for Dumbledore. I think the first actor nailed it, and I think the second actor has lost all of the kindly, grandfatherly aspect that we came to expect from him. He doesn't smile enough, he doesn't have that twinkle in his eye, he just doesn't ring true to Dumbledore, I don't think.
Harry did a good job with what he was given, I think. Being the main character he had a little more time for character development, but I think it would have helped to include the scene in the book where Ron asks Harry to stop taking out his temper on him and Hermione, and to include the part about Harry feeling like Voldemort was possessing him, and the scene where all the kids are talking about it and Ginny tells him that he couldn't have been. That would have make the character a little more whole and a little less "just angry" all the time.
And aside from the fact that Bellatrix and Tonks didn't get nearly enough screen time, I wish we could have seen more of them (and did Tonks even say "wotcher" to Harry? She totally should have), that is about all that I think I have to say. Hope you all enjoy the movie too.
EDIT: How could I forget! They made Cho the traitor, the one who told on Dumbledore's Army. Originally, I was going to say that I hated this. It was probably the one plot tweak that I liked the least/disliked the most. I just read someone else's review where she went through the reasons why they probably did it, and it boils down to this, I think...it's more theatrical for Cho to do it. It's not necessary, it doesn't make it better than the way that JKR did it in the book, but it is a more theatrical way for the DA to be disbanded. I don't really think that Cho's motives make much sense, as she wanted to be in the DA to help avenge Cedric, and even if her mom's job at the ministry was at stake (another change from the book), I doubt she would have volunteered the information when asked. I supposed they could have forced her to drink Veritaserum, but that only makes you tell the truth, not walk the teacher over to the Room of Requirement and let her in. I don't know, like I said, it's more theatrical, but to me it still doesn't completely make sense.