New Firing Squad Episode Two Interview|FiringSquad:

FiringSquad: The newly revealed Team Fortress 2 is a lot different than the game that was first shown at E3 in 1999. What caused the change in art style to a cartoony 1960's spy look?

Doug Lombardi: Starting with the characters in TF2 we had a set of goals that we wanted to accomplish, ranging from making sure that you could identify a class and its abilities from just it silhouette, to allowing gamers to form an emotional attachment to the class they most liked to play. Once we felt like we had a set of characters that were meeting those goals, along with all of the goals in between, we started building a world for them to play in that was complimentary.