I can tell you right now "slow leveling" didn't make FFXI an 'epic mmo.'

Leveling speed has very little to do with how epic an MMO is. It's the content that makes an mmo epic, not the total time it takes to reach the highest level. In FFXI, reaching max level was just the beginning there was so much stuff to do.

FFXI was epic because the size of the game was epic, the storylines were fun, the quests were fun, the events were fun, Combat was fun, the game was balanced, etc.