Community is the simple answer. Most games have it... but none to the extent EQ ever had. Reputation as well, people remember how nice or rude you were. If you made a mistake and owned up to it. Like trains, not to mention no other game really has trains today
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And then of course running around in cloth armor for levels upon levels because you actually had to work on it for an upgrade and it was meaningful, whether it was camping it or a long quest line for the upgrade. Sure it may have been small, but it was still a big deal.
Epics were hard ! And still are... I still remember my first epic on my very first character. Getting all these friends that then called in favors from their friends for the final fight.
Community, reputation, teamwork, challenge, etc etc etc... good thread but unfortunately probably beating a dead horse. Other than player/fan base stuff... developers don't care.