It's the whole package.
You *FEEL* powerful. 8 enemies your level and you're half dead? No problem, pop an inspiration and charge. Other MMO's, you better have a team with a dedicated healer to take those guys on. The powersets feel like they've got Oomph, there's plenty of variety, and being able to color your powers makes a difference in how powerful you feel.
The UI is MASSIVELY customizable. Color, window size, text size, position, opacity, the whole nine yards.
None of this "7 powers max on the bar" bullshit.
Your powers you got in the beginning of the game still matter at level 50. "PEW PEW PEW WHAM!" is your buildup.
The QoL stuff.
I have yet to see an MMO that lets a ton of different people of all different levels all effectively play together. That sidekick system really blows other games out of the water.
Enemies don't feel like reskinned stuff. And the groups stay, just have better stuff, all the way up.
No "gear dependent" shit. You don't have to worry about not having the latest raid gear, and no arguing over who should have got that drop.
Your looks aren't overlaid by your gear.
I don't know, it's the whole package. Where other games just slavishly copied the main MMO's, CoX feels like it went its own way and discovered something good.