Few things I disliked about this when this first came out, and still dislike today
I still dislike how puzzle elements were presented to the player in this game, the way it's all done ruins any sense of amazement and wonder that the first game delivers in spades. I am of course speaking about the fact that whenever you trigger a button or super heavy co- button you have to be standing in a specific spot and looking in a specific direction when pressing the button to see what the button did and to see the change happening.
In Portal, buttons don't do anything until you look at the location of what the button changed. Walls and Stairs don't form until you look at them, letting you actually view the animation/process of how they change/form. In Portal 2 it very frequently all happens off screen, out of view.
This is my one and ONLY problem I have with Portal 2, I know it's explained WHY they changed it in the commentary mode (iirc) and I know they had good intentions, but I just feel it ruins some of what made the first games atmosphere magical, seeing the world evolve around you is more rewarding then hearing the world evolve around you.
Everything else is top notch, amazing stuff, no complaints about anything else.
Most of this is happening in the parts of the game before you actually begin escaping with Wheatly.