Although you can get trolls or even get raided by a bunch of trolls, automated moderations helps a ton with that. Stuff like word filters come in clutch!
I remember back in the day as I was building a few gaming communities, it was tedious and time consuming trying to acquire members and retain them day by day/week by week.
When you get excited about questlines..you know, that nostalgic World of Warcraft experience when you pick up a quest that takes you on an adventure? Yep. That..but for your community experience.
I suspect the payment processing argument is a convenient red herring. Adult sites and their transaction service providers process payments all the time, some at incredible scale.