I guess the greates weakness of Portal (2) is that its replayability value is very low. I did play Portal 1 twice, but the second time was just for fun; I still remembered the solutions, so there's little new to be found. I could imagine that this is particularly hurtful for coop play in Portal 2, because once you completed them, there is probably little motivation to play them again.
I wonder if it were possible to create a decent random level generator (perhaps for Portal 3). Not, of course, to replace the excellent manually crafted campaign levels, but just to get a couple of minutes of fun out of the game every now and then. As long as they were "good enough", this could significantly increase the long-term value of the game, particularly in coop.

