(For the record, my standard policy is that I don't think change is intrinsically bad; after all, I still have the original book to read. But I'm more critical of bad changes than I am of new ideas that don't work, because the writers don't have the excuse of "Hmm, I didn't think of that".)
Book 4 followed the trend of getting a bit darker, so I was surprised that this film was the funniest so far. I wouldn't count it as a comedy as such, and there were still serious parts to it, but there were also a lot more things that made me laugh. I think the best line of the film was Hermione's "I am not an owl!"
Speaking of Hermione, I think that in many ways this was her film, even though she's not the eponymous character. At first, I was a bit dubious about the actress; it makes sense to keep the same cast (where possible), but she wasn't delivering her lines with much conviction. However, that improved quite quickly, and I did like the way that she was basically kicking Harry and Ron into action.
On the general theme of casting, I think that the only real flaw in the film was that Dumbledore's accent kept slipping, often within the same sentence. (And apparently the actor has never heard the words "pensive" or "sieve" before, since he pronounced it as "pen-seeve", thus missing the pun.) I think some people were surprised to see "Trigger" (from "Only Fools and Horses") in a different role - I've seen that actor in a couple of other roles, most notably the sarcastic doctor in a medical comedy, so it didn't faze me in the same way, but I'm glad to see him doing well.
Then there's David Tennant; I liked the 5 minute "Dr Who" episode that they showed during "Children in Need", particularly for the way that Rose reacted to the regeneration, but he didn't impress me very much - he just came across as pretty wet. That said, the 6th Doctor started out by trying to strangle Peri, so I'm willing to cut him some slack and give him a chance to prove himself, but it didn't bode well. I've heard that he was previously in "Casanova" (on the BBC), but I never saw that, so my completely uninformed opinion is "Ah, that will involve a lot of poncing around and looking pretty to impress the female audience then". However (yes, I was going somewhere with this!), he did display more range in this film, so that gives me a bit more optimism about his future work.
Coming back to my comment about the film being abridged, the part that I specifically missed was Harry's underwater count/threat to the merpeople - that was one of the most gripping parts of the book, which really made me feel as if I was there (aided by holding my breath). But as I say, I still have that scene elsewhere, so it's not too bad. However, I do think that they could have afforded an extra second or two to show the other handheld dragons; that seems like a decision that was made to save money, rather than time, and I'm less sympathetic towards that.
As a related issue, there are some things which were present in the film, but weren't made as blatantly obvious as they were in the book. Specifically, I'm thinking about the reference to Barty Crouch Jr using an unforgivable curse on the aurors Frank and Alice Longbottom. If people are paying enough attention, they can make the connection (to Neville's parents), but it doesn't get spelt out (if you'll pardon the pun). Similarly, Dumbledore explains the graveyard ghosts as "priori incantantem", but doesn't elaborate - if you know Latin, you can probably figure it out, but it's not explicit. As for Karkaroff, did we actually see him disappear (or be absent when the Durmstrong boys left)?
And speaking of the ghosts, I think that was the best scene in the film, and certainly the most moving.
On the flipside of all this, there were things that the film did better than the book. I don't remember any mention of Neville being an experienced ballroom dancer, but I did think "Hey, go Neville!" when he was doing well for himself, so that was nice to see.
I was also impressed by the first challenge, where Harry came up against the dragon. In the book, Ron comes up to Harry afterwards and says something like "Wow, this really is dangerous - you could have been killed out there!" I had much the same reaction when I saw it on the screen. Right from the start, when the dragon's tail smashed down next to Harry (as he entered the arena), I thought "Woah, if that had hit him, he'd be dead right now, and it was sheer dumb luck that he wasn't standing there". This was sustained while the dragon was chasing him around. By contrast, I remember watching "Star Wars Episode I" in the cinema, which included the pod race. As I watched that, I thought "I assume that this is supposed to be thrilling, edge-of-your-seat suspense, but I'm not feeling that; maybe it's just because I know full well that Anakin will survive". Coming back to Harry, I knew that he would survive too, but it was still exciting. So, I think that this is a case where the film complemented the book (although I suppose this could be revealing my own lack of imagination).
A related aspect is that I was surprised by the dragon's movement; I've tended to visualise them as quadrupeds with wings, so they'd move like Pegasus. But as I watched the dragon climbing across the rooftop, I thought "Hmm, it's moving like a lizard. Which, in fact, I suppose it is." That's probably not a startling insight to the dragon-fans (I would say "dracophiles" but that has unfortunate connotations in this context), but it was new to me.
The seating arrangements also seemed to be changed a bit from the book, e.g. having everyone on the towers mid-river. As I watched the film, that struck me as being a good idea, although on reflection I don't think that the second/third challenges were really much of a spectator sport.
By contrast, I was a bit dubious about the Beauxbatons girls doing their "synchronised sighing" when I saw it in the trailer (partly because my initial reaction was "Hey, it's only supposed to be Fleur who's part Veela"), but that actually made more sense to me when I thought about it. There's a line in the book (when they're at the campsite), where someone (Ron's dad?) says "It's always the same when a bunch of us get together, we can't resist showing off". So, with that in mind, I can well believe that the visiting teachers would get their students to rehearse a flashy entrance.
Finally, a couple of character points. Snape actually came across as surprisingly sympathetic in this film. I found myself rooting for him in the scene where he was supervising the homework session, and Harry/Ron just would not shut up, so I was grinning expectantly when he pushed up his sleeves. As for Cedric, bazzalisk mentioned that he didn't come across as quite such a decent bloke in this film, which I'd agree with. However, I think that made him more human in a way, particularly since he had his own circle of friends (rather than just being linked via Cho and Amos), so that made his death more of a blow.
Anyway, all in all I'm looking forward to the 5th film.
And now to bed - sitting cross-legged on concrete is less comfy than you might think :)