I know this is a little old. But, I figured I'd put it out there in case there are other Portal 2 users who miss the game (maybe through user-generated levels).
I enjoy the game Portal 2. The thing is, as much as I appreciate the gameplay involved (as compared to an FPS or zombie game), it's a linear storyline. There's no open world for you to do things in (like TF2, if you know what I mean). You go from point A to point B and then the game is over. Mind you, there are achievements that Valve built into the game that give you an added incentive to gain those achievements. But, still, this is one of those innovations that ended up as a one-off.