The aspect of the podcast that I want to bring to your attention is an example of great game mastery. Currently, the podcast is a series of actual play episodes with a WOTC GM (Chris Perkins), the Penny Arcade guys, Scott from PvP and Wil Wheaton. Not only is it hilarious (with much foul language), it contains the work of a great GM. Listen carefully to what he does.
Knows the rules - quickly! The players want to try something and Chris very quickly has an answer. He doesn't let the rules get in the way of the game. Where necessary, he involves the players for the text on their powers and abilities, but only to help them know how to use those powers.
Evokes without cliche. NPCs have voices and attitudes, but they aren't crappy cliches like a bad 80's fantasy movie. His rendition of a bar-wench and of a dwarven warrior are great, without making me cringe.
Lets the players shine. This crowd has a particular sense of humour. Chris gives the players enough material to work with to make the snappy jokes, the one liners, and the moments of exultation that they enjoy about their games.
Keeps the adventure moving. This game doesn't bog down. If a combat takes a long time, it does so with interesting components. It's not long and boring, it's long and interesting.
Makes NPCs multi-facted. NPCs almost never attack with a single attack type, they use everything available to them. This is partially (I suppose) to advertise to the listener about all the great features of D&D, but it makes for great listening. When the NPC has a turn, it's not always with the same weapon or spell, it's one from a wide variety of actions.
If you are a GM and have time for podcasts, I strongly recommend listening to this and critically analysing the GM techniques. You'll learn a lot.
And to Chris Perkins, well done. You're a great GM.