Well, it's true that portal didn't get any dlc made by valve, but that was mainly because it was a small 10$ game to see if it would get popular.
Still, there was quite a lot of community maps and mods around it, some of which are almost at the same level of quality as the game itself. But then again, single player games don't have that much play value, even with loads of custom maps.
Portal 2 on the other hand, will most likely allow people to make custom co-op map, which will be amazing. Also, with a netcode in there, modders might even be able to squeeze even more people in there.
