Portal 2 looks like an actual sequel, not a rushed rehash like L4D2. The other actual sequel that Valve made was Half Life 2. You see where I'm going with this. I am 100% confident that they can produce gameplay that will make Portal 1 feel like a child's drawing. However, Portal is much more than just the gameplay, there are many little elements that clicked in a marvelous way and made the first game such an absolute joy: The art design, the (black) humour, the clever writing, the ambience, the "nerdy" atmosphere, that god damned song in the end... Can Valve also hit all those key points for the sequel and improve them ever more? That remains to be seen...