Consider this a friendly notice that, should you be in the mood for some Team Fortress 2 soon, you're going to have to wait on a lengthy installation process. Upon launching the game, a content-conversion prompt will appear which can take up to half an hour to complete on even a fast machine, according to Valve. What gives?
This one-time process is necessary for upgrading Team Fortress 2 to SteamPipe and while it might seem annoying now, it should be more than worth the lost time. SteamPipe is an improved content delivery system that promises "faster uninterrupted downloads, quicker boot times and map loads, and easier distribution, installation and management of mods."
Those are certainly things I'd like to see in TF2. Let us know if you've seen these gains first-hand. I would, but the process hasn't finished for me yet.