Ease of payment, when SA instituted this barrier a young troll intent on disruption had less of a problem coughing up 5 dollars for an account, and more of a problem finding a means to pay this 5 dollars. This is not the case for today's youth. See Xbox live and twitch donations if you don't think I'm being fair here.
Scale-ability, which you directly touch on. Enforcing an upfront cost is a very real value calculation one must make prior to having extracted any value from the service and something most people my self included are currently adverse to. This directly impedes a communities ability to scale.
I can completely relate to your idea that I wouldn't pay to post on the forums. For years I had friends telling me to sign up and I always just shrugged and responded MEH why would I pay for that sort of thing? I didn't understand (at the time) the point or the reason for doing this. My (and many people's) value structure was in-congruent with the owner of SA.
In retrospect though it was more to do with me not understanding how to use generator functions to better control memory bloating.