Every class is so tightly focused on doing its thing that TF2 feels like nine different games fighting each other. That's bewildering at first, but it's a joy to watch characters this beautiful smash each other to pieces while you learn.
Stuff like this has an intensifying effect on your moment-to-moment experience: you feel, see and comprehend the game world in Technicolor. It makes all the relationships instantly clear and the importance of your actions explicit. In short, it makes everything you do 300% cooler.