I've more or less got the hang of the game (I still can't play a spy worth shit, but I'm finally a functional engineer), and I'm struck by just how... well... completely unlike the Left 4 Dead games it is.
There is no story, there's very little backstory, and what backstory there is exists more for comedy. Ditto the characters (they have their own personalities, but there's no limit to how many can play at once). And while Left 4 Dead hand-holds you into the game, TF2 has a relatively steep learning curve. There are no tutorials, no sandboxes--there are tips, but they show up randomly.
Nope, you pretty much jump in and start playing and learn by dying a whole lot. Reminds me of old-school platformers that way.