One of the most interesting bits revealed is that Valve originally toyed with the idea of not using portals at all. "What if [the Portal franchise] was always about introducing a new puzzle element that you're going through?" asked Valve's Erik Wolpaw. "It's about Aperture Science, and now you're going through this new testing track with this new element." The team tried using both paint (which is in Portal 2 as gel) and a gravity gun as central mechanics, but every tester wound up wanting the portal gun back (and who can blame them?).
Speaking of paint, Wolpaw revealed that there is another kind of gel in addition to the propulsion and repulsion gels. This third gel is apparently "sticky" and PC Gamer postulates that it may enable players to walk on walls, though Wolpaw noted Valve is still "working on it." Check out the full interview for more Portal goodness.