Snape ki… no wait, wrong film. -_-
In general, the whole movie felt way too condensed and consequentially disorientingly fast-paced. One of the praises from critics for the HP:TPOA was that the director, then new to the series, decided to cut out a lot of things in order to achieve a more faithful and compelling portrayal of the main storyline within the time constraint. However, as the book grew longer and longer (though HP:TOOTP was shorter than HP:TGOF), even that approach seems to have reached its limit. For example, when Dolores Umbridge gave the inaugural speech at the year-start feast, in the original novel Hermione gave a detailed explanation of why the speech meant that the ministry would be interfering in Hogwart's operation. Somehow every single one of the details got omitted in the movie, and viewers were left with no choice but to scratch their head and mutter “Well, I guess… but why?”—that is, unless they read the novel before watching the movie. I can see how that would be more or less expected, seeing what a bestseller the books have been so far, but nevertheless it seriously flawed independent completeness of the film.
There was also a blatant omission of one scene: In the novel, Sirius gives Harry a half-mirror that Harry could use to communicate with him. After Sirius's death, Harry tries to contact Sirius using it, hoping Sirius would somehow respond. There is obviously no response, and that's when Harry finally realizes Sirius is truly dead and nothing could bring him back. I think it is an important plot device that enables Harry to get over his godfather's death and to move on, and it would also have had a dramatic effect had it been featured in the movie.
In other news, Cho Chang got served bearing the role of the sole traitor against D.A. Marietta, Cho's friend and the real traitor, didn't even appear. Perhaps the screenwriter thought Cho could be dispensed because she assumes no other significant role afterwards than just avoiding Harry in general (at least that was the case in the next volume). Although I don't really like the character of Cho, I feel kinda sorry for her for that injustice. -_-
Now acting: There were both (pleasant) surprises and disappointments. Harry was supposed to fully swing between all sorts of positive and negative emotions (anger, frustration, joy, loneliness, sorrow and of course, greenhornish crush-love), but Daniel Radcliff didn't manage to express the wide spectrum of emotions but ended up mixing them into a lukewarm and awkward character.
As for Ron, there wasn't enough chances for his character to shine, but whenever there was a chance, Rupert Grint did a fairly satisfactory job. Especially when Harry poured his frustration onto Ron, who was taken aback at the unexpected anger, Ron's awkward surprise was very naturally portrayed, neither dumbed down/lukewarm nor unnecessarily standing out.
Emma Watson's acting (as Hermione Granger) was neither outstanding nor horrible, but she wasn't given much chance either, just like Ron.
Mad props go to Imelda Staunton: Her Dolores Umbridge was exactly as portrayed in the novel. Everything, from her usual sweet front to the hidden real personalities such as cruelty and arrogance. I think I'm torn who to pick as the best actor/actress, between her and…
Matthew Lewis, who played Neville Longbottom. Neville's character in the novel walks on a tight rope, between the usual awkwardness/clumsiness and the determination/defiance that stems from his past, and Lewis's acting digested the delicate balance very well, much better than expected.
Luna Lovegood's character, played by Evanna Lynch, was more or less as expected, but it left one thing to be desired: Being a weird girl with a dark past, Luna was supposed to be both comical and mystic, but the latter was largely missing in the movie. I don't blame the actress though, because the timing and setup of her lines indicates that it was the director's intention to simplify her character.
Alan Hickman's Snape. Two words. Stoic comedy. It was a great refresher. XD Oh yeah, same goes to David Bradley's Filch. XDD