Well, okay, I suppose I'd better be more specific.
To start with, I'm not completely "purist" -- just as there are things I like in the Lunar remakes despite preferring the originals, there are things I liked in the movie despite preferring what I'm used to. I wasn't expecting Michael Crawford or Sarah Brightman. Both had moved on by the time I first saw the musical (1991) and unless their voices were dubbed in I really wouldn't expect them 15 years after they left the show. Though I was surprised at the intensity of anger I felt when I read that the movie was all ready to go with the two of them when Lloyd Webber backed out 13 years ago due to his divorce from Brightman....but that's neither here nor there.
The biggest disappointment by far was the singing. The person who played the Phantom -- a vocally demanding role -- simply couldn't cut it. His voice was rough, it wavered, there wasn't enough vibrato, and some of it had been rewritten to accommodate his voice. I realize that they were undoubtedly prioritizing acting over singing, but really, with all the people who have played the Phantom over the years on the stage, they couldn't find one person who looked good on camera too? (Personally, I would've been very intrigued had Robert Guillaume, who took over when Crawford retired, been there, but I suppose that's no more likely.) If the singing had been less central to the role, as it is for, say, Meg or Madame Giry, I would've understood, but this was a bad choice. One of my favorite dramatic moments, originally at the end of act I, was ruined because he couldn't hold the note well. I think that's what it was, anyway.
I also found Christine disappointing. I probably could've forgiven her not very pretty face -- I thought she looked too old for the part, actually -- and lack of expression had her singing been more on, but she tended to scoop into her notes a la pop singer, which made me wince every time. I was again very surprised that apparently she's experienced on the stage, and am somewhat inclined to think that the producers were so astounded to find someone of the right age and approximate other qualifications to play the part that they didn't look very hard for alternatives, though of course I have no idea whether that was the case. Her part too sounded like it had been rewritten to accommodate a lack of range; the title song omitted a verse and consequently ended lower than the original high E.
Lest you think I'm completely down on the singing, I thought Raoul's was quite adequate, as were Meg and Madame Giry's (though Meg in particular looked a bit odd when singing her solo in Angel of Music). The other parts were fine too; I thought Carlotta's voice was the best done and was rather amused to find that it was the only one that was dubbed. I've never heard of Minnie Driver before, but it sounds like she at least can sing -- why wasn't the Phantom dubbed? :P
Other things...lyrics were changed, of course. I have no problem with this in principle, of course, and many changes were fully appropriate. Some were less desirable, though. At least one nice rhyme was ruined by a change which added nothing (sorry, the specific escapes me). One reference to the Sûrete (the French police) was removed, presumably for the ignorant American audience. Eh, well. Some other changes were different from the Broadway album but matched my memories of one of the performances I saw and of course I have no problem with that.
A place where the lyrics were not changed and probably should've been was the Masquerade scene. The lines are referring to all of the fascinating costumes...flash of orange, flash of puce, swirling yellows, etc. Yet the scene was practically black and white! My mother and I were both deeply disappointed that the fascinating costumes from the stage production were not retained, though their designer, Maria Bjornsen, was given a credit. We'd both looked forward to seeing them more close up on the biggest screen we could find (a Cinerama dome) and instead it was the Black & White Ball [an annual party in SF, for you non-Californians]. I understand the probable dramatic reasons for this, to make the Phantom's red costume stand out, but it really was a shame and seemed a bit of a copout. A couple of subtleties were lost from this scene, such as Andre and Firmin frightening each other with identical skeleton-in-cape costumes, Christine being whirled away by increasing numbers of similar dancers until being reunited with Raoul, and the cymbal-playing Monkey Girl mimicking the music box monkey.
On the other hand, I was pretty much okay with adding the Phantom's jabs at the cast to that scene -- it makes more sense on the screen to have him do that in person rather than by notes. But it was really a shame that the rehearsal scene was omitted, since the modern character of the Phantom's music was therefore lost (Piangi's inability to sing a whole-tone scale, foreshadowing the 20th century's atonalism). Instead, we saw the opera audience utterly failing to "get" his music, reducing his genius to bad-fanfic-level delusion.
One thing I thought was appropriate and neat is that actual explanations for effects such as Carlotta's losing her voice were put in rather than it just being "magic" as it is in the stage production. I wish I could remember the book better on this point, since I remember both him gaining some power from the music box and that he used mechanical stuff, but it doesn't matter, I guess.
However, there were other explanations which I didn't think helped things. The scene when the young Giry met Erik (the Phantom) is the main one. I suppose the idea is to focus on his deformity and what it drove him to do early on, but, as with the music, it turned him from a genius to a madman. (In both the musical and the book, as I recall, Giry saw him as a man in a cage, perhaps while traveling with her husband. More importantly, he'd been the architect to the Shah of Persia or something like that, and was in the freak show as much for his demonstrated and amazing intellect as for his deformity.) This both made her personally responsible for his actions to a far greater degree -- and one incompatible with her level of involvement -- and, more importantly, made them contemporaries in age...which works for the stage production but not for the rather youthful Phantom of the movie. Madame is at most a few years older than Erik in that flashback and has a daughter Christine's age.
Another contradiction is the graveyard scene. I will grudgingly concede the right to change it to a stroll through a cemetary, because the dramatic transition I loved in the stage version just wouldn't have the same impact here. But the swordfight...the heck? Genius or no, I just don't buy that someone who's lived in an opera house his whole adult life will be able to wound a young nobleman (presumably trained by private swordmasters). He can fence purely from seeing stage fighting but he can't sing?!? Give me a break.
Another weird thing was the business with a ring. First, it being a crystal engagement ring was rather blatant product placement -- you all saw the Swarovski storefront, right? (They did the chandelier.) It was just odd that the Phantom takes the ring that Raoul presumably gave her, then gives it back to her, then receives it from her, then ultimately leaves it at her grave. Isn't it strange to do that with a ring that another man gave her originally? (Incidentally, in the book I think he dies a few weeks after Raoul and Christine leave.)
Overall, if it hasn't been plain, I thought that the character of the Phantom was severely weakened. Showing the murders themselves didn't help; it was much easier to feel sympathy for him when they were offstage. Perhaps it's just my advanced age (since I'm probably older than the Phantom was intended to be) but I didn't find him at all attractive or compelling and therefore couldn't really empathize with Christine's reaction to him. Not to mention the implausibility of him teaching her to sing (ha!). I don't think he ever played the pipe organ onscreen, either, removing another dramatic moment.
I read a short article on the movie after I saw it, which mentioned that the director saw the story as being about Christine's sexual awakening and therefore her age and that of her two suitors had to be younger. I didn't like this, prude that I am, but more importantly I thought this was a spurious reason to make the Phantom younger. Furthermore, he'd intended to make the Phantom a " bad boy" type, attractive in a dangerous way, but perhaps because of my bias toward intellectual attraction (i.e., to the intellect that was removed from his character) I felt this totally missed the mark. The selection of a rock singer for the part was intended to work with this, and indeed when I heard the opening of the title song I thought of Steve Harley's performance of it, but that rapidly faded...I've been over that before. I really felt that an older character would've worked better, as well as made his lack of companionship the more poignant. (Phyco, I only saw it once, but it really looked to me that he was supposed to be quite young.)
What else...Carlotta felt overdone to me, even though she's a stereotype. The managers were fine and the comic bits ("scrap metal") were fine too. Some of the visuals were stunning. Some scenes were unnecessary, like Raoul's near-drowning. It drove me nuts when scenes would switch from speaking to singing in the space of a breath. I liked the theme of the rose. Breaking the mirrors was interesting. I wish I'd seen Christine's birth year. The prologue and epilogue were nice -- I recognized Madame immediately. Moving the chandelier fall was fine. Drat, I just don't remember what else I was going to say. I suppose this is enough, though. :P
(As a postscript, I no longer remember everything in the movie that I commented on.)