Still very short singleplayer with great story. Much easy challenge than Portal 1. Max. graphic level of the Source eng.
Portal 2 was developed towards a mainstream game, which is very easy to finish (i only take 6 hours, someone may be faster than me). The game introduced more interactive elements but very not challenging (may be i already educated by portal 1, made me "skilled"). I hope there would be some community maps pack that really challenging, without the flood of hints, to be provided through steam. There is a challenge made but the related achievements was removed, made 100% more easily (instead co-op became another factor)
However, to be honest Portal 2 had a great story, except you had to listen and read carefully instead of using the brain on progress only. Portal 1 lack of real story, the player character voluntary finish the tests and then aimed on survival. However, this time the player character seems had a clear aim and background story, especially about she is a clone or not. (based on a scene of Portal 1)
back to gameplay, there is co-op mode which give bonus to this game. However it also a tactics, which you had to advertise you friend to play with.
For the graphic, the Source engine was a old one. However in some scene the game use dynamic light heavily, despite you did not had a shadow. I don't know it is pre-rendered or not but it is eye-catching. The liquid was preformed well, but because it was old, and indoor environment, there is lack of more eye-catching scene. Properly the interactive elements (such as gravity, angle of reflection) consumed hardware capacity and had to give up the maximum graphic of the Source (such as HDR)
To sum up it is worth to buy for its community map and co-op mode, but for the main game itself, put it in the bargain cart