I love one thing that probably put a lot of people off about Killing Floor : the repetiveness. I can relax and listen to some music and know there will be no surprises. I know what I am doing and the only thing that changes is my teammates. I know how to play the perks to the best of my ability and usually feel like I am helping my team without having to try very hard. Because everything is generally the same, this allows me to have fun playing challenging custom maps or enjoy playing with "bad" ( uneducated ) players.
Music is another thing I really enjoy about Killing Floor. Not the stock music, but my own music I play instead. I don't spend a lot of time listening to music outside of playing games, and there is something really fun about listening to Bassnectar while killing zeds.
The last thing i love isn't that obscure, but I love feeling like my team wouldn't have made it so far if it wasn't for me. Whether that means playing medic and keeping everyone alive or keeping our camp spot from being overrun by choking off a spawn point or simply knowing when we need to run and being able to kite out the rest of the wave. This doesn't happen all the time, but when it does it feels good man.