Ok, so I had a thought a moment ago. Now stay with me here...CO is one of the best MMO's I've ever played.
There is only one other MMO that kept me playing longer than I have played CO. Is length of time spent playing a game the sole reason to judge 'best'. No. Absolutely not.
But that other game and CO both share something that has kept me around. Customization.
Every MMO I've played is fundamentally the same. And that's not a bad thing. It's a genre I fell in love with a long time ago. I don't need great deviations from these fundamentals.
But how I judge (I hope it goes without saying that this is my point of view and opinion only) if a game is any good, are the extras like customization.
Are there games out there where I have preferred combat, or the questing, or some other standard aspect of MMO's over CO? Absolutely. But all that stuff means nothing to me apparently without more to do.
I was surprised by this myself. But thanks to the character customization in this game tied in with the usual suspects, I have been playing CO longer than any other MMO, save one.