The game dynamic has been made to sort of aid the player... Because of this, it is easier to get puzzles to work once you have solved them. Some of the puzzles themselves are more challenging, but once you've figured out how to solve it, you don't have to spend quite as much effort trying to make sure things line up - the elements in the game sort of know where to go.
I was actually very surprised about how much back story there is throughout the game. I was happy with GLaDOS just mocking and taunting me, but Valve has outdone themselves here... Not only is the back story great, but the way they let it unfold for you as you're going through pre GLaDOS era tests is really great.
If you've read reviews of the game, you probably already know about Wheatley and Cave Johnson -- these characters are fantastic and add a lot of life to the game.
The graphics are amazing and the test chambers are huge. I'm amazed how well the game runs (granted I am playing on an Intel Core i7 970 with an ATI Radeon HD5700 video card, but even with all the graphics settings maxed out, there is no lag or jumpiness... The game is flawless and beautiful.
The "Cooperative Initiative" goes above and beyond and really adds a lot of replay value to the game.
To say that I am pleased with the game would be an understatement. Suffice to say that I enjoyed playing it and I will enjoy playing it again and again. You should too.