I'm not massively impressed with the movie - i would have to see it again to pinpoint exactly why, but i'm not sure I see your point about "pointless" changes to the action scenes. Some are made to look more spectacular, but most were justified by the medium or storytelling needs E.g.:

- Rowling was able to tell the story of the last duel as she did because she wrote from harry's pov, and isolated him and Voldemort that way, made it about them by focussing Harry's thoughts on him. On screen, they really had to get rid of the crowd and have them alone.

- The shrieking shack (a lot of complaints about that): they got rid of it because they needed a location at which Harry could easily witness Snape's death while the audience could see Harry at the same time (and not in close-up reaction shots from under the cloak, tne scene was too long and they needed us to see
Harry's reactions as it played out, not cut back and forth.

- The fiendfyre had to go. The kids had already found an alternative to the sword, and using the fire required not only to explain Hermione knew of that and there's no doubt he horçrux is gone, but also that it's uncontrollable and can't be risked. Exposition that would have broken the rhythm for little purpose (the sort of things a bad director like Columbus used to do early on) otherwise they created a plot hole (why didn't Hermione use fienfyre againt Nagini?).

- Nagini and Neville. That one is very simple. They couldn't just drop Ron and Hermione again in the climax of the movie (it already happened during the forest scenes, they needed to bring them back in the action), and they needed to delay the death of Nagini so it happened right before Harry killed Voldemort, which worked better on screen than having Nagini die before the duel began (as of course in the book it's more about Harry's thoughts than a proper duel).

- Molly and Bella. Well.. There was no more to it than that, really. I think a lot of people are disappointed they didn't inflate that scene, but THAT would have been pointless. Neither the movie Molly nor Bellatrix deserved more, unlike the book Molly, a far more important character.

- Lupin and Tonks. Again, not a change, and there's a very good reason why they did not show this fight or the death of the twin on screen, the very same as Rowling's: she killed those characters, and did it off-screen, so their death came as a shock to Harry when he learns, bringing real personal meaning when Voldermort says Harry is sending his friends to die in his place. The audience needed to learn at the same time as Harry. The movie showed more of the fighting, but it couldn't show that, especially not when we can't follow what goes on in Harry's head. I thought those moments rather well-done, hardly "blink and you'll miss it" as i had expected from comments. Both the mourning scene with the Weasleys and the couple's shot were pretty powerful visually and emotionally.

- The chamber of secrets was a good way to give Ron-Hermione some screen time, and to give them a moment alone (the movie audience really wanted their big pay-off kiss).

- The Ravenclaw scenes had their purpose in the book but none to the movie. The confrontation with Snape was really good as they did it. Not that it's better than the book, but for the movie it was. Again, the movie Snape and Harry could use the face to face moment the book characters didn't need.

That said, i'm not too sure why I also found the movie a bit underwhelming... I was annoyed by the editing, but I doubt it's just that.