I find myself (somewhat reluctantly) agreeing with Kaos Arcana about Neverwinter Online.
First, the differences: 1) no sub option (that's alright, I can limit myself to an average of $15/month expenditures on the game), 2) no superhero travel powers (which is a HUGE difference, however the detailed environment helps mitigate this), 3) limited character creator (nothing to mitigate this really), 4) fantasy rather than superhero genre, 5) choose a single class with skill trees rather than mixing two powersets for the archetype.
Now, the single biggest similarity: combat flow. I've tried around a dozen non-CoX MMOs, and either their combat was way too static and slow (TSW, even with the "action" elements) or fast (CO, in which non-SVs drop like flies, and DPS is an absolute dictator). I hope the combat flow I've experienced (up to level 40, quest content), stays with the end-game, even with the perception that tanks just slow a team down (sounds familiar to some opinions in CoX). I find the combat visceral (in that I'm always involved doing something), and satisfying (in that enemies are tough and still vanquishable with the right tools/skill). It is the combat flow which is the primary reason I returned to CoX multiple times, and ended up staying to the end. The character creator, travel powers, etc. were good reasons, however since most of the game time was combat, that was the primary consideration. NWO may finally be the MMO which meets the same standard.
Of course, if any of the CoX-successor projects gets to open beta, or if CoX itself is brought back, I'll likely leave NWO pretty quickly. Until then, I'm @wyldhaunt in NWO.