The two of them discuss some of the groundbreaking aspects of Portal such as the remarkable length and how it just plain makes players happy.. In a three part series, there are more words than the entire script of Portal, but it’s an interesting read:

The secret of "Portal"'s success, you see, is that it may well be a 2D game in 3D clothing. Not in terms of how it's actually designed, but in terms of the values in the design: its spartan life. "Portal" is a reminder that a three-dimensional game can be simple. It's a refinement of what "P.N. 03" ? the GameCube game that looked like a 3D action game, played like a shmup ? was designed to express. It's a victory for those like me who enjoyed "Crackdown" because ? not despite ? its neglect of the "GTA"-style side missions that clutter so many open-world games. Ultimately, it's a reclamation of 2D game design values. Keep the concept to a simple thing and do that thing right.

Just beware: to keep the argument interesting, the authors had to consume some pretty serious amounts of Haterade.