The fundamental problem I see is that it kind-of 'corrupts' gratification. When I make a post, or do a vote, or write a comment, I dont want to think about the monetary value of it. There is 'value' inherent in creating/sharing interesting content and thats all I want to think about.
Adding money to the mix makes it a distraction from a 'pure' experience and takes away value than adding it. Ads do something similar - but imo ads are less distracting than putting a monetary value on an upvote.
Still this is very interesting experiment. Would love to follow updates on hn.