Who would have thought that cake could so easily motivate people?
The gameplay is unique and simplistic, yet so much can be accomplished by creating two portals to travel through. The puzzles are creative and engaging. Some of them can be solved in unique ways that the creators probably never even imagined, which is awesome.
The most amazing aspect of the game is the story. It is hard to think of a puzzle game that has a story, much less a terrific one. You find yourself not only wanting to play the game for the puzzles, but to find out what happens at the end.
Lots of the negative comments about the game have resulted from the game not being particularly long if you just beat it once. However, the game really shines for me in the extra challenges. Some of them are insanely difficult and make you think. Even as I write this, I haven't finished all of the challenges, but I go back every once in a while to try some of them.
Overall, Portal is one of the best puzzle games of all time. It brings a lot of unique elements for a puzzle game, such as a first-person perspective and a good story, and combines them into a solid puzzler.