I don't get what could be taking so long. They came out with L4D, tweaked it a bit. Then they copied it for L4D2 and haven't given any support for L4D2 since they released it. They made TF2 which was just copying 1/3 of TFC and making skins and calling it a game. If no progress has been made on a new engine, HL3, or Episode 3, then I figure Valve is like 5 people.
Is it that time consuming to find bugs in their games, releasing patches of nothing, and spitting out update notes that say they fixed bugs that they didn't fix?