It has ultimate replay value - you could make millions of maps, all of them would be different. Sure, I get bored of Civ and play other games for a while, but I always come back, because its always fresh and new.
It has the perfect balance of strategy. Its not TOO in-depth, where the learning curve is insane and you have to check the manual every few seconds to see what this does. Once you pick up on it, you don't forget.
It lasts a very long time. Games will last a week, two weeks. Some hate this, but I love it - it keeps my dedicated. It makes me try harder to keep my empire afloat, because I've just put 30 hours into its creation.
It has good difficulty levels. I am playing on Monarch right now, because I want a relatively easy game. I could play on Emperor, and have a decent challenge, or I could go to Diety and fight for my life. If you find Diety relatively easy, then you need to stop playing.
In short, it has everything!
EDIT: How did I get my way to Civ? I always had an interest in history, and a slight interest in politics. I played Age of Empires II from a friend, and I loved it. Interesting in these "strategy games", I picked up Civ2 (Civ3 was coming out shortly, but I didn't know) and I loved it. In fact, I only really played one game of it, but the game was extremely fun - I was an isolationist power, Greece, doing my bidding while staying put on my island. I loved how I could choose my fate, choose to be an empire, or isolationist, or anything. Then I got Civ3 when it came out. Never looked back.
