Caution: May Be Habit-Forming!!!
Hmmm, just about everything. Great (and very addictive!!) gameplay, no-frills graphics, and more customizability than you can handle. And even though I get tired of this game sometimes and quit playing it for months, I always seem to come back to it eventually.
The music gets kind of boring (though you can turn it off), and the original version was NOT multiplayer (though multiplayer was added later -- I eventually got Civ2 Multiplayer Gold). Otherwise, I can't think of much else.
The Bottom Line
Don't play it if you have to get up early, because you may be up all night. Start playing at 9 or 10 PM, and next thing you know, it may be 4 AM. An all-time classic!