Overcome it's shortness, and you have a solid and unique physics based game.
Three characters are presented to the player, the Thief, the Wizard, and the Knight. Each has different abilities that allow the player to solve puzzles in different ways. What's interesting about Trine is that the player can switch between any of the three characters at any time (provided the character still has health). Each character has his or her own life and energy bars. The Thief can shoot arrows and use a grappling hook to reach high places or to swing. The Wizard can create crates and planks (usually used to help reach something, press a button, or as a counterbalance) and can levitate and move objects. The Knight can perform melee attacks with his sword and hold up a shield to protect himself. Many puzzles can be solved in different ways with different characters. This was of particular enjoyment to me because while trying to solve a puzzle I sometimes die, leaving me the two other characters to solve it and forcing me to rethink my solution.
The music and sound effects are appropriate and nice, and the voice acting is great. The characters don't speak much, but it's nice when they do, and the narrator does a superb job at telling the story like a fairy tale. The writing is nice, and the story more than sufficiently supplies motivation for the game.
Adding more depth to the game, Trine also features experience which can be collected. Each level has a set amount of experience, and when collected, experience is applied to all characters equally. When the characters level up, the player is able to pick skills to improve. In addition, equipment can be found in the levels. The player chooses which character to give the give the equipment, providing the chosen character with a bonus.
You can also play Trine with a local drop-in cooperative mode that lets two or three people control all the characters at once. Three players makes the action a little too busy and demanding, and removes the ability to swap between characters entirely, which makes some of the obstacles a pain to get by. Two-player co-op feels like the sweet spot, since either player can still swap to the third character at any time to maintain that puzzle-solving flexibility that makes the single-player so much fun. And two characters can pull off some tricks you can't do in single-player, like the wizard giving the thief a levitated ride across a bottomless pit on a conjured platform. Still, this adds replay value to a great game.
In conclusion, Trine doesn't last long(around 5 hours), and while there a few things keeping it from such lofty heights, you should absolutely play this game. It's smart, it's fun, and it's well worth your time.
Final score: 8.0/10