I think it works because, from your very first moment in the game, you're out there taking down bad guys (or good guys!). You feel like a potential superhero or supervillian from your very first step. In other MMO's, you start out so weak that a butterfly farting in the field next door sends you flying and you spend your first week collecting flowers or some other time wasting nonsense. Virtually everything you do in CoX helps with your progression whereas most other MMO's seem to be designed to keep you logged in for no purpose other than to make the numbers look good on paper.
Having said all that, there are a lot of frustrating things about CoX, few ever really liked those endless convoluted caves where you could spend half an hour hunting down that last well hidden npc, but even that was addressed to some extent with those little red markers now showing their location. I can't help wondering what direction this game would have gone in had the plug not been pulled.