Was it's complete lack of content obsolescence. Nothing in VoX ever became irrelevant, Oroboros made low lvl missions challenging for max lvl characters going back to do missed content, arcs, TFs and GMs received updated rewards first through resipes then through merits to keep them important as new content was added, all endgame progression was either cumulative or parallel with the incarnate system both building off of all prior progress and existing alongside IOs rather than replacing previous endgame upgrades like in most other MMOs, the updated difficulty settings allowed you to tailor the content to your power level rather than trivializing it. It completely avoided the treadmill feel off so many other MMOs you just ran in place and all new end game content made everything you've done before pointless. This was what gave the game such staying power and high player retention because progress actually mattered and your favorite content always relevant.
I've been thinking about this a lot lately in the context of why most other MMOs felt so unsatisfying and realized I hadn't seen many others mention this aspect of CoX which is annoying because it should be the industry standard.
