This is the first of the films where I really felt that I was "missing something" from not having any exposure to the books. There were so many characters briefly splashing across the screen (with only minimal explanation in-film) that I had to go spend a good long time after the movie poking around in Wikipedia for the back-story on the likes of Nymphadora Tonks or Bellatrix Lestrange. Sure, there seemed an awful lot going on "behind the scenes" with the Ministry of Magic, the Death Eaters, the various lineages, etc. in the previous movies, but that sort of obscurity is understandable from the standpoint of the Hogwarts students, but Order of the Phoenix just bubbled over with stuff that needed fleshing out.
One fascinating bit of back-story that did make it into the film was the snippet of flashback of Snape's past where Harry's father was being portrayed as a jerk, a bully who tormented the young Severus when they were both Hogwarts students ... no wonder Snape had such a bad attitude towards Harry when he (as enough of a visual near-clone of his father that Sirius Black at one point calls him "James") arrived in Snape's classes!
It's fairly remarkable that they've managed to keep together such a wide-ranging cast over the course of 5 movies, but the aging of the actors is really beginning to stretch the ability to "suspend disbelief", and one has to wonder just how "too old" the kids are going to look when they get around to the 7th film in 2010. Heck, even this one had a bit of a "Mod Squad" (to be a geezer and reference the cheesy Aaron Spelling series that ran for five seasons in the late 60's and early 70's featuring mid-20's characters who posed as teens to assist the police) feel to it! Daniel Radcliffe is already branching off into other projects, and it's not unfair to wonder just how much focus the main actors are going to bring to the last two films (especially if they need footnotes as much as this one did!).
This all is, of course, not to say that Order of the Phoenix was a bad movie, it was quite interesting, enjoyable, and well paced ... it could have, however, easily filled up another hour of screen time to have been more "internally consistent" with the various sub-plots! Of course, that's the take from somebody who has not read the books. As I understand it, most of the hard-core HP fans are happy to have the abbreviated characters appear (with them being able to fill in the details from memory), and are bitching more about big chunks of the books which have just been dumped. This sort of puts the producers in a "damned if you do, damned if you don't" position, as it's no good to produce a film that's unintelligible to the non-fanatics, but they also don't want to alienate the hard core HP base. Personally, I'd hope they'd go with producing the best possible movie adaptation, even if that meant trimming away some of the bits and pieces which only mean anything to the addicts of Rowling's books!