Here are two of my main reasons for liking CoX more.
I tried CO as well. the graphics turned me off, but then so did the artwork of the Teen Titans cartton as compared to JLU or Batman TAS. I adjusted and came to love the show.
That didn't happen with CO. I enjoyed the freeform powers, but that was just about it. The game had no heart. It was just a hastily slapped on re-skin of a Marvel game with equally no heart.
Here, the community, despite the insanity of the average forum, is warm and welcoming once you get into the game. You feel wanted, like you belong, no matter how new you are. No other MMO or multiplayer game I have ever tried, short of the original Neverwinter Nights, has accomplished this. I have more friends in game than I think I've ever had in real life. Many of them better friends than any of my RL ones. That's shifting slightly because I've been talking the game up for seven years, so now they want to try it too.
and yeah, the journey. There are still plpaces in CoX I haven't been. Eden, Recluse's Victory, I barely touched the Shadow Shard back then. I took 4 years to get my first 50 and never worried. Never cared "it's taking too long". Conversely, I tried STO at start and made 50 in a week. I proceeded to get very bored with it.
In short, there is no "one thing" that makes this game so loved. It's all the pieces, put together just right. And we'll be here forever because of it.