In the book, the Tournament is merely a “friendly competition between the three largest European schools of wizardry”. All tasks were controlled by the Ministry, who had “worked hard over the summer to ensure that, this time, no champion will find himself or herself in mortal danger” - and if they don’t die, there’s no real danger at all, because these are wizards, who’re able to heal all possible injuries very fast. In the movie, however, the Triwizard Tournament is very dangerous and not controlled at all.
Let’s go through the three tasks and compare the book and movie version:
The First Task
The dragon is chained up in both the book and the movie, but in the movie, the dragon breaks free, and chases Harry around the castle, climbing on the roof. If this happened in the books, the task would have been interrupted by the dragon tamers! Also, the book dragon doesn’t know Harry’s the champion, so even if she was unchained, there’s no way she’d follow Harry and leave the eggs unprotected in front of the audience. The challenge here is to get the dragon to leave her eggs at all, not to climb on the roofs alone with a monster, while the teachers sit there doing nothing.
The Second Task
In the movie, the merpeople are enemies, who are trying to drown Harry. In the book, they’re friends, who’re ensuring everyone’s following the rules and returns safely to the surface. They only attacked Harry when he tried to rescue more than one person and then, they watched him in order to make sure he wouldn’t drown. They’re even smiling at him once he’s finished the task.
"Harry, you prat," said Ron, "you didn't take that song thing seriously, did you! Dumbledore wouldn't have let any of us drown!"
…or be eaten by a dragon, for that matter. But the film directors apparently didn’t listen to Ron.
The Third Task
"We are going to be patrolling the outside of the maze," said Professor McGonagall to the champions. "If you get into difficulty, and wish to be rescued, send red sparks into the air, and one of us will come and get you, do you understand?"
Yes, we understand. There’s no mortal danger this time either. Too bad the directors didn’t understand that.
The book has a maze filled with animals, while in the movie, the maze itself is enchanted - but this time, I think it is more due to time limits and repetitions than lack of understanding the books. I’m fine with that. What the directors failed to understand this time is Harry’s personality. In the movie, Harry hesitates between saving Cedric or taking the Triwizard cup. In the book, Harry immediately goes for Cedric, and then he offers Cedric to take the cup for himself.
These changes cause a lot of other problems, too.
In the movies, it seems so silly how Dumbledore allows this madness to go on at his school, especially to a fourteen-year-old, while not permitting the students to go to Hogsmeade without permission from their guardians. The magical contract that forces Harry to participate isn’t properly explained - no wonder people think Dumbledore was evil or mad.
It also makes Ron seem like a worse person than he was (again…), since the viewers are left wondering why on earth Ron would get jealous when Harry becomes a champion.
"Jealous of what? He wants to make a prat of himself in front of the whole school, does he?"
“Make a prat of himself.” That’s what Harry was most scared of once his name was chosen. Not “being brutally killed”. There was no danger of permanent injuries, but you could gain the money and popularity Ron always wanted to have.