A huge portion of The Goblet of Fire is dedicated to the "sillier" nature of Harry Potter: the sports, the melodrama between the kids, and the general ridiculousness of the magical world itself. The remainder of the film is dominated by the darkest and most powerful events we've seen yet. On one hand, the sillier stuff works. The kids are teenagers now, and a fair amount of humorously awkward melodrama makes sense. It's largely funny, and works surprisingly well.
There's some weird stuff going on with the cast this year. For one thing, every teenage male main character has extremely long hair for no apparent reason. It's really distracting.
Ron seemed to pick this year to act like a suspicious unbelieving jerk towards Harry, without any real reason. Why is Ron suddenly so jealous of Harry? Eleven-year-old Ron was quick to sacrifice himself in the Wizards' Chess game for Harry's sake, but here he suddenly doesn't give his best friend the benefit of a doubt? A normal teenager would probably respond with this type of jealousy and suspicion, but Ron has proven himself to be better than this in the past. It doesn't really make sense, and it's not compelling drama nor humorous comedy. It's just an annoying distraction that could (and perhaps should) have been replaced by the same great trust and friendship that we've seen between Ron and Harry in the past.
One notable change from the first three films is a new composer. John Williams' replacement, Patrick Doyle, takes cues from Williams in the tone and style of the music, but crafts a strong score that feels slightly different, yet totally fitting. Harry himself is given a specific musical theme that's woven into the score at various moments of personal drama. Without a doubt, it's one of the most emotionally powerful themes of the entire series.
As in the first two films, much of the story feels like it's jumping through a list of fantastical life-threatening situations. This can be a problem, as it means that the entire movie is basically a highlight reel of the most important events from the books.
The Quidditch World Cup is really fun to see. Prior to this, we've barely seen the magical world outside of Hogwarts and Diagon Alley. Here we finally get to see a bigger picture of the entire wizarding community. It really helps fill in some of the gigantic gaps in Harry Potter lore and paint a better picture of the world we're seeing.
The Triwizard Cup challenges are mostly nothing more than action scenes. They are, however, visually stunning and extremely well-directed. Harry's battle with the dragon is so amazingly real-looking that you can't help but feel totally enraptured by the tension of it all. The mermaid scene is perhaps less impressive, but it's still got good elements of fantasy horror and a healthy amount of imagination. The final challenge, the maze, is practically pointless. The danger within the maze is never revealed (aside from a few cantankerous bushes), leaving only a vague undefined threat that doesn't really catch the audience.
When The Goblet of Fire gets it right, it really gets it right. The return of Voldemort is terrifying, horrific, and powerful. It's also a hugely important event that isn't resolved by the end of the film, thus finally breaking the general rule of self-containment held by prior films. On the other hand, when The Goblet of Fire gets it wrong, it just feels stupid. After dealing with the super-important events of the last film, why are we supposed to care about something as pointless as the Triwizard Tournament? This is a problem in the original book as well, but that doesn't excuse it.
The saving grace of the "pointless" events—the Quidditch World Cup, the Tournament, and the Yule Ball—is that, at the very least, they're entertaining to watch. They get a bit boring on repeat viewings, but they're still reasonably fun. The comedy is highly ridiculous at times, but not quite stupid. It works for the most part.
The filmmakers can't entirely be blamed for the problems with Goblet of Fire; they did just about the best they could with the source material. And, in the end, what's there is still good. It's not as good as Prisoner of Azkaban, but it's definitely a fun movie with a shockingly dark and game-changing ending.