Having played both quite a lot, I believe the skill ceiling of TF2 characters is much higher. Plus, rocket and sticky jumping are just too much fun to give up (sorry, phara and junkrat just don't compare). I like the old-school movement abilities that held over from the Quake Engine. I'm also not a huge fan of timeouts and cooldowns for everything.

Finally, I like all the different modded servers out there and the ability to play casually when I want to just mess around.