Multiplayer civ is probably a different story, but the single player version provides no incentive to keep playing. It was fun the first time through, you see everything there is to see within one or two games. You'll hit 90% of the tech tree on a single play through, you'll get most of the units, you'll conquer cities, etc. And what happens when you win? Nothing. You get a pop up box, basically saying "you win!"

It's just boring to me is all. I don't have any story motivations to make me want to conquer the evil "empire x," I don't have new units / abilities / powerups to look forward to, etc. With the exception of the names of the generals you're playing against, one play through of civ is exactly like the next. It feels empty, and lifeless. Almost like you're playing a campaign editor. All the parts are there. . .you can make four unit x's, and have them fight two unit y's. . .but there's no reason to do so.