Here is the new system just revealed by Blizzard - Followers. You may have a powerful character, but we could all still use a powerful person to come along with us; maybe you don't want to play with friends on multiplayer, so you could just use an AI companion!
Portal only had a single player mode with a short, but awesome campaign; Portal 2 has a single player campaign with double the length (about 8 hours) and a multiplayer campaign with about double that length (about 8 hours). I have taken screenshots from the coop multiplayer mode - quite a few of them, too!
I have played Portal 2's single player campaign and the entire thing was amazing; now, I'm playing the coop campaign and it's going extremely well! I never imagined that multiplayer of any kind would work in Portal, but it does! Here are a few screenshots from the first level of the single player game that I took.