There's a huge universe to explore, fight, always something new to do or in some new way. (I've more-or-less mastered air-raiding a planetary base at this point, I fear.)
But more than that, there's always something new to glitch, and this is at least half the fun. The only other game that I own that even comes close to being as fun to screw around with and try to play wrong is Outpost, and BC:M is vastly better for this sort of thing. I could write a hundred pages on how to reproduce and benefit from BC:M's bugs if I wanted to, but I don't. About the only thing that still eludes me is getting the game to fire pulsed laser beams out of the monitor.