More Quidditch would be cool, but you can tell that Quidditch already taxes their CGI f/x capabilities (or budget) to the limit. It's probably the most expensive f/x scene they have to make, and they still can't really do it justice. I can understand that they'd always have incentive to minimize the Quidditch screen time.
(Consider: Quidditch is probably far more difficult to get "right" than the dogfights in Return of the Jedi. Quidditch requires facial expressions, arm and leg movements, clothes flapping, synchronization with live crowd shots, and then on top of all that, it's a sport faster than soccer. Just imagine how hard it'd be to render normal soccer using full CGI. Modern video games are startlingly good at this, but they've been doing nothing but sports-specific game engines for maybe two decades to get that far. Heck, a modern game like FIFA'09 probably has a bigger R&D budget than an HP movie )
Given that CGI is hard and expensive, you'd want to judiciously spend your time on a few critical non-human creatures that you can't do any other way. Hence they reduced Hagrid's herd of hippogriffs to only a single Buckbeak (who, BTW, was quite excellent), minimized Sirius's dog form (ironically, it looks like they did not just use a real dog), did only a passable job for Lupin's werewolf form, and used real snowy owls, cat, and rat for Hedwig, Crookshanks, and Scabbers. It's still much easier and cheaper to train a real animal than to fake it with CGI, and the real animal will look better. Dementors ended up looking like the flying squids from Matrix
My own Quidditch fantasy-sequence came while reading the World Cup sequence in GoF (which I have not viewed yet on my 2-DVD set). It's a truly silly mix of modern NBA, FIFA, and Cirque du Soleil showmanship.
- Ireland chaser #1 with Quaffle, rushes goalward on a 2-v-keeper break
- Chaser #1 draws keeper out, passes into space, slightly below chaser #2's path ...
- Chaser #2 deliberately slides off his broom, drops into Quaffle's path ...
- ... catches Quaffle in mid-air, twists, dunks it with two hands through a goal hoop ...
- ... hangs momentarily on the goal hoop, as his lower body swings down-and-up the other side like a pendulum ...
- ... his riderless broom has done a split-S, and is diving vertically behind the goal ...
- ... with body horizontal, he lets go of goal hoop, falls away from goal, with slight residual rotation toward upside-down-ness, face gleeful, left hand reaching blindly backward ...
- ... his broom dives into his hand's grasp, he no-look grabs it, rotates his body slightly in midair, and presto he's astride his broom and flying again (in a nearly vertical dive) ...
- ... levels out, grabs the bottom of his jersey with both hands, pulls it up over his head like a hood ...
- ... flies off with arms spread wide to each side, shirt covering his face like a mask, midriff exposed, as his teammates fly parallel and shoulder-bump him ...
- Ireland 10-0 Bulgaria
Of course I realize they'd never film this -- it would be, ahem, too spectacular for the rest of the movie
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Eric / Gilmoy
The book(s) put such emphasis on mature Patronuses having a specific, recognizable animal shape. A stag galloping/cantering is much more thematic than the amorphous "deflector shield" effect they showed. I suppose this was a CGI limitation -- they didn't have the budget to do a stag galloping around. It also fits with #2. Ultimately, I saw it as a dumbing-down to cater to non-readers, so of course we readers would find it disappointing. We're special that way
It was mildly amusing to see flying Dementors getting knocked out of the air by Harry's Big Ball of Patronus. I'd trade it for the stag, though.
2. Moony, Padfoot, Wormtail, and Prongs
It looks like they had to cut out all this discussion for time constraints. That decision did de-emphasize the significance of "Prongs" being a stag, hence it's at least consistent with #1. Maybe they couldn't fit both #1 and #2 in, so they both got clipped.
3. Harry responding to Sirius's invitation
The point of the book scene was that Harry accepted Sirius's invitation enthusiastically, and Sirius knew that. It emphasized to both of them (and the reader!) that Harry valued their relationship -- which, for Sirius, must have felt like water after twelve years of desert. The movie scene cuts Sirius off just as he's awkwardly backpedaling, thinking that Harry has rejected him. I thought that was slightly cruel -- it deprived Sirius of one joyful thing.
4. Sirius "whoo-hoo!"
Gee, considering that Hermione and Harry are on a rilly tight schedule, and that they probably need Sirius to escape in secret, aren't they broadcasting an awful lot of noise in a Sirius-like voice from locations that are not the highest room in the tower? Good thing dementors are deaf as well as blind, eh ...
Harry: Is this really the time for it? Oi! There's a rescue going on!
McGonagall: -- pursued by a host of dementors -- by the way, they can --
Dumbledore: -- fly, yes, we've heard --
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Eric / Gilmoy