A big theme from the session was the emphasis placed on playtesting often and including feedback in the development loop:
“Playtesting is probably the most important thing we did on Portal,” Swift continued. “Sit down and watch people play your game. Don’t just have them send you reports: It’s not going to be as valuable. You can find out what you’re players actually want by watching them. Adjust gameplay to what players look like they need, and adjust story to enhance what players are already feeling.”
Swift revealed they began testing the first room from the moment they started at Valve. For example, the first room initially featured a “shimmery force field” and players didn’t understand what it was -- so they changed it to glass. Throughout the game they proceeded to make the world cleaner and simpler to help teach the players what they need to know.
Also discussed were the difficulties encountered when sculpting the end-game. Head over to Gamasutra to read the remainder of the article.
Best of GDC: The secrets of Portal's huge success (Gamasutra)