Since it creates a tree structure, you can wipe out entire armies of bot/spam/otherwise accounts by following the vouches up the tree.
* dead online communities
* highly-invasive, government-mandated "prove you are a human" requirements in order to participate in online communities
The intriguing part is that I think it works against scaling. The incremental cost for me to use the 500GB of free space on my disk is $0, but someone scaling a bot farm has to buy all their space.
Real people tend to have a lot more idle capacity than optimized, scaled businesses, so any kind of proof of idle capacity seems like it would disadvantage bot farms.
I’ve also thought that proof of collateral spending would be a good system. For example, you buy groceries and the store gives you a token saying you spent $X of real world money. Those tokens help show you're not a bot. Keeping that system honest and equitable would be extremely difficult though.
Maybe schools could give kids tokens for attendance. It sounds kind of dumb, but who knows.