#1. The level of customization - the map generator, custom PNGs, weapon power, crate percentage, turn delay, and on and on and on!
#2. The atmosphere - the music, the art style, the voices, the crazy weapons...they all come together so beautifully.
#3. The physics engine - wind, gravity, momentum, arcs, bounces...deep, yet predictable enough to allow for high levels of play.