It took me many years to finally have the willpower to push myself to get past them and finish the game. Some levels (especially in the second half of the game) are a bit too lengthly and sometimes feel a bit overwhelming. + The ending is fantastic which involves an awesome boss fight, lots of craziness, cake, and the best credits song I've ever heard. + Quite a few easter egg type rooms with some humerous drawings and writing + GLaDOS makes a lot of funny jokes and remarks throughout the entire game which keeps things interesting and enjoyable. + I personally feel like the story is what makes Portal "great" especially near the end of the game where you are trying to escape from the facility and kill GLaDOS a robot who controls everything in the lab who tries to kill you multiple times.
These puzzles continue to get more difficult as you get near the end by including obstacles like turrets but the game really makes you think outside the box. Others require you to redirect energy balls with your portal gun to activate various switches. Most consist of getting from point A to B, some require you to move cubes on top of buttons to open doors. + The puzzles are challenging but not "too difficult". You can even use your momentum from falling to slingshot yourself across a stage by placing portals beneath you. You receive a Portal Gun which lets you create two portals that you can pass through at any angle to get to various out of reach places or to quickly navigate from one place to another.