I think user-interaction. Obviously having more users helps as that leads to more interaction, but that is not all.
If we look at this games peak, the following things were happening:
- the top orgs were having regular write ups, we had much more content in general.
-top fights were often hyped up and they had meaning/value.
-we had regular major tournaments where orgs would collaborate together, one such idea was the Olympics
-orgs regularly competed against each other in super fights.
The forums were fun, but at one point they became very toxic as well. I know as many people (or more), who avoided them, due to that toxic atmosphere.