I love how its free, i play this game far more than any other $50 games i bought at the store.
I love the mass driver, and the marauder.
More than that I love owning MD noobs with a dretch. ;p
I also like how the teams are almost opposite. Its makes players change up their playing style.
Its open source.
Each alien/weapon is potentially a death machine, depending on who's using it.
I also love how players can build their own bases, it adds a new type of [ i cant think of the word

] to the game.
My favorite thing is that there are no hacks released for it yet, i hate playing in 'hackfest' games like CS:S. (this is when people shout hypocrite >;p )
I also love my nifty little custom HUDs. (mybe I'll post a SS some time) ;p