The acting has really improved beyond recognition. The trio have become more and more settled into their roles with age and maturity, and aduts such as alan rickman and maggie smith were brilliant once again. I loved the imperonation of umbridge, and much preferred this portrayal of sirius to the one in the 3rd movie (gary oldman did a great job, and they really built harry's and padfoot's relationship. Luna was perfect. Only criticisms (and even then, it's rather personal/opinion-based ones) are Mrs Figg's portrayal, and the new-look dementors
-Favorite scenes that I wish had made the cut (or fingers crossed, the extended-edition DVD!) include: Petunia's "remember my last howler", harry trashing dumbledore's office after hearing about the prophecy, Sirius' two-way mirror gift to harry and it being smashed but never used, and a longer and more detailed flashback to the marauders' era via the penseive rather than snape's direct memory.
-I loved the many obvious r/h hints and the slightly more subtle h/g hints
-I wish bellatrix hadn't cast avada kedevra (and used another curse with a red beam as in the book) before sirius fell into the veil, as this may have given the impression that the curse (rather than the veil) was the cause of his tragic death. As noted above though, they did a good job in building sirius' relationship with harry (to make up for his very brief role in the 4th movie).
-During the thestral flying scene, it would have been really cool to show a glimpse of flying from a non luna/harry perspective ie the appearance of flying in thin air!
-I understand the reasoning behind making Cho (rather than Marietta) the traitor, though I'm glad it was revealed later that umbridge used veritiserum on her
-Although I knew that Quidditch would play no part in the movie, it would have been cool to hold the "Weasley is our King" match offstage.
-As in previous movies, I loved the comic relief provided by fred and george (extendable ears, apparition, "the big bang"!, etc), and was really glad that hermione's claim that ron has an "emotional range of a teaspoon" made the cut!
OVERALL, A WONDEFUL MOVIE AND EASILY THE BEST HARRY POTTER MOVIE SO FAR, BUT MORE IMPORTANTLY TO DIE-HARD HARRY POTTER FANS, A WONDERFUL PRELUDE TO DEATHLY HALLOWS