DEFCON's visual simplicity and darkly radiant beauty detach you completely from the horror of what you're doing. It's replaced by the emotional rollercoaster of caring deeply for every little line-drawn icon on screen. After deploying your units in the early DEFCON stages, you never receive any replacements, so every loss is permanent.
That, coupled with the brutal suddenness and galling power of a well-executed enemy attack, make DEFCON instantly engaging and almost unbearably tense.
A concentrated attack from two countries at once, orchestrated or coincidental, is impossible to survive. That's still fun at LAN parties, but DEFCON's atmosphere is so oppressive that being picked on in a net game, alone in a darkened room, is just plain unpleasant.