The class system. Other shooters may have loadouts or custom appearances but they all still play the same. In TF2, every different class plays completely differently and that adds to variety both on a surface level and makes the matches so much more dynamic than other shooters, with players on both teams changing guys just for the ♥♥♥♥ of it or to counter the other team or to better accentuate their own. In other games you're just doing the exact same thing the whole time, you know?