*SPOILER IF YOU HAVEN'T SEEN THE MOVIE YET*
It doesn't really matter why they turned into animagus. For people who haven't read the books, it's a moot point. They don't know any better and it wouldn't ruin the story for them.
Yes Sirius is dead, but as Loony said, things that are lost to us always find their way back to us, even if not in the way we expected. So I'm pretty sure Sirius will find his way back into the final book. Personally I'm in denial about Snape really killing Dumbledore. Only 9 more days
The things I would have liked to have seen, or been done differently:
The death scene was pretty unextraordinary;
Kreacher mumbling more about filthy half-breeds and crap under his breath;
Harry talking to Kreacher through the floo network;
The Swamp that Fred and George left that no teacher would remove
The statues coming to life in the Ministry;
The dementors and the death eaters appearances changed from the last movie;
Harry didn't feel nearly enough full of hate/resentment towards the world, and his last scene with Dumbledore in his office was unremarkable;
Seeing Neville's parents and Professor Lockheart at the wizard hospital - the scene where Neville told Harry about his parents was crap.