I think the problem with Civ is, and always has been, that the more proficent you become at it, the easier it is to realise that you're going to win the game through whichever victory condition you're aiming at.
The more proficent you get the earlier in the game that tipping point comes. You being to realise earlier and earlier that you're hitting the unwritten stages for victory.
Once you've hit that point however, it can become extremly hard to finish the game. I've quit games after taking down 3 other Civs with another 3 left on the table. It's only a matter of time before they go down, the challenge has gone and the game, to some extent is over.
Part of this is due to the AI's inability to improvise, part of it is that there aren't enough short term objectives to keep you occupied along the way.
However, with things like CivFanatics GotM and Hall of Fame, the game gets a bit more longievity I feel (although I have never actually submitted to either).
The thing is, I don't see a way past that "tipping point" issue. Once you know you've won, the love goes out of the game. It's hard to push past it sometimes.
But that doesn't mean you can't replay the game, there is enough in there for you to set up your own victory conditions and ignoring those of the game itself. Or trying to beat your own acheivements, the quest for an earlier space race or 16th century modern armour always keeps me going.
And I'm sorry, but there is no feeling like seeing the map change to your colours entirely.
I don't care how many games you've played, watching that on a replay is always utter win!
