The ubiquitous argument about how "radical editing" was unavoidable given the 875 page original book is essentially a nonsense. The problem is not that so many plot lines were shorn (they were, but by in large that's OK and not the reason the film doesn't totally satisfy), but that the core story is so sloppily told with so little care to motivations.
The movie assumes that most of the audience knows the story and will forgive story changes which will allow the director to delete characters (Dobby for instance) yet stick some characters (Kreacher) in for a scene or two but omit the only REASONS they were in the BOOK in the first place.
Not to give away plot revisions, which are in fact relatively minor - ranging from who proposed the use of "The Room of Requirement" to who exposes its use (a particularly nasty change), to how the supply of "Veritas Serum" is supposedly used (or the missing explanation necessary as a result of the change as to why it didn't work) - but a simple list of what is left OUT of the movie from the book (details which need NOT have taken significant screen time if any additional at all - NOT major sub-plots) may serve as warning against disappointment:
*The coaches which the Thestrals pull (they're mere dogcarts now). *Quiddich (playing or bans even referred to). *Confiscation of brooms (requiring alternate transportation to distant points). *Lessons (of any kind except in the D.A.!). *House Elves (except for two brief, non-plot appearances from "Kreacher"). *The Invisibility Cloak. *The Marauders' Map. *Pensives. *St. Mungo's. *Ghosts AND poltergeists. *Cleaning of the Headquarters of the Order. *The *concept* of "Secret Keeper". *The Hour Glasses and the House Point system (why not, since they omitted the scoring from the contests in ...Goblet of Fire!?) *Dumbledore's Office (and access to it or lack thereof). *How the D.A. was named (or how Umbridge heard about it). *The enchanted Galleons communication system (or any substitute). *Any explicit blockage/monitoring of student communication including the Flue Network and Owls! *The Quibbler *Any censorship of outside publications. *Umbridge's racism (or WHY her actions against the centaurs are so provocative). *Attacks on Hagrid AND/OR McGonagall *Outside O.W.L. examiners. *Why Filch likes Umbridge so much. *Any hint as to why Neville may be such an important character - or might have been even MORE important. *Magical swamps. *ANY attacks on Umbridge at all or student resistance to her regime (except at the *one* time Rowling specifically made it clear the Weasley's at least would NOT - when it would interrupt important student activities - specifically when they are sitting for O.W.L. exams!) *The elimination of *any* member of the Order of the Phoenix from Hogwart's grounds *except* Dumbledore so there was a REASON Harry couldn't call on them for assistance in a crisis. *Who MADE "the prophesy," why it is important & why it is a secret. *How the Dementors came to BE in Little Wingins. *Any celebration of Umbridge's departure (her *rescue* is not even mentioned) and most stunning of all, *HARRY'S SCAR* (it literally isn't ON his forehead until it makes one brief, suddenly darkly drawn in appearance late in the movie)!
OK, readers may well wonder how they COULD tell the story without these things (except for the final fight scene, this film eliminates more actual MAGIC from Hogwarts than Umbridge does!), but still, over all the film works. Rowlings provided a rattling good allegory of right wing governmental repression.
Imelda Staunton's Umbridge is an evil joy to behold (even if her best scenes were eliminated), and Evanna Lynch's sublime Luna Lovegood is a true find (even if the REASON for her character to be IN the book - her publisher father - has also been eliminated). Most of the student cast is growing wonderfully into their roles - from the three leads to (especially) Neville Longbottom.
The utterly silly (frequently noisy!) plot changes surrounding The Room of Requirement or even the astoundingly stupid and time wasting taking down of all the portraits on the Hogwart's walls can't undercut what remains a fast moving fun story for reader and non-reader alike. Those who know the book will be aware how much better this movie could have been. Those who don't won't have to worry too much about it - despite the odd plot line left hanging and action curiously unmotivated - director Yates keeps things spinning along so swiftly, most won't notice 'till long after the interminable crawl at the end is a distant memory.