Portal 2
Portal 2 is better than Portal in every way. I must say that this game must be extremely hard to make, because you have to achieve a progression that makes sense for the player and the sweet spot of difficulty and complexity. Portal 2 brings everything from Portal with one exception, the plasma ball was replaced by lasers. The laser can be used to power up something or the reverse logic, power off something by blocking off the laser. The game is twice as large as the previous, with more challenges than before. The beginning is a recap of the previous, with the same puzzles meant for training the player. When we reach half of the game new mechanics with three different types of gel are presented.
Everything in Portal 2 is larger an richer than in Portal. You can jump higher and farther away. The environments are larger. The challenges are more complex. There are two evil AIs. The plot is more detailed. The game is harder than before with more than double the number of puzzles. The puzzles require spatial vision. You have to have the ability to see things from multiple angles: back, front, up, down, left, right, to see the exit. The levels that show the backstage parts of the facility from Portal are much larger now. Now we get to see tanks, pipes, chasms, assembly lines, press machinery, elevator shafts. There is much more details than before. In HL2 episode two Valve added large scale destruction. In Portal 2 the destruction could rival the destruction seen in Crysis 2 and 3.
In HL2 I missed a full developed Gordon Freeman, including voice acting. In Portal I did not. The human placed inside the chambers could be anyone. The voice acting in Portal is so good that the sarcasm, laughs, sadism. All are able to let you imagine a character without you ever seeing it in game. The AI from the previous games has many more lines. They were able to find a balance between being cute and being machiavellic and cunning. When the new gel mechanics are introduced to the game a new character is introduced. It's J. K. Simons! His voice was perfect in Portal 2.
Having played both Limbo and Portal I see a common concept. How do you defeat drones if you don't have weapons? By taking advantage of the level itself. For ex: use the portal gun to redirect a laser to hit a drone. That's the kind of depth that the two mentioned games have. The level is more than just walls, floors and blocks. Letting the player see through the level design is a great feat. If there is a game that could be called 4D it's Portal 2. There are the three spatial dimensions, plus one additional time and mind dimension.
Someday we are going to see Portal 3. There are people in Valve who want another Portal game. The problem is that Valve has a decentralized system that requires a large number of people supporting the idea to make it happen. If or when Portal 3 happens, how do you make an even larger, user friendly and enticing game to surpass the previous?
