"but we need some enforcement mechanism to remove scams and illicit material"

Yes, we would need the ability to actually remove posts, not just hide them.

"but we would need some sort of automated "proof-of-brain" and protection against people controlling hoards of sock-puppets)"

That could be accomplished by limiting the number of votes any member can make within a given time period, and perhaps even start charging steem to menbers who go over a certain limit. It would also be a good idea to limit memberships to only 1 per person. This can be easily accomplished in a number of ways, such as through the tracking of IP addresses, Google 2FA Authentication, and so on.