Portal was new, different and very entertaining, but that's just about everything it was. Portal 2 is all that, but also delivers an engaging story, colourful and memorable characters (I'm in space!) and even more of the cheery/dark humour that made its predecessor so unique.
I thought the games learning curve was very smooth, which is very nice since its gameplay can get very complex at times. I remember that I had a much harder time playing through some of the original Portal's levels, and that usually ended with frustration rather than perseverance. This time the puzzles are not easier, but they are better thought out and designed. Most are challeging and very satisfying to solve.
The main story is not that long, but offers just enough depth to suck you in without disturbing you from your task and from the simple pleasure of watching and listening to your robotic friends/foes. Portal lacked that depth and that made it definitely less enjoyable, but Portal 2 offers a really nice balance.
Humour still plays an important part of the interactions between characters. GLaDOS doesn't pack as much punch, but Wheatley, Cave Johnson and some other minor characters more than make up for it. It has been a while since any game made me laugh this much. The writers really put a lot of work and creativity in the dialogue, and for that I'm grateful - hats off to Valve.
The visuals are standard (using the original engine) but unlike some other games, it doesn't affect the game in any way. Portal 2 is the kind of game that does not need sophisticated visuals or music to be good. It's gameplay is so effective and intuitive that you won't even notice what it lacks in visual style. There are also some discreet jokes that pop up throughout the single player (a great Half-Life reference earns you an achievement) plus the environment contributes well to set the tone of the game.
Co-operative testing is a big novelty and is just as enjoyable as the single player mode. Of course, it doesn't benefit as much from the story and the humour, but the new challenges and experiences it offers are so entertaining that you will have just as much fun (if not more) playing with a buddy.
Portal 2 has an interesting replay value. I had a lot of fun hunting down achievements, and co-op with a mate will make you crave for more. There has also been an official announcement from Valve that FREE downloadable content will be coming to Xbox Live soon, can't go wrong there can you?
Portal 2 is more than a game; it's an experience. It can't even be defined as a genre. It is unique, and for that it deserves to be played by everyone.