1. It doesn't solve any problems of group think, because if pg and the Y Combinator folks think the system is already tilted toward a certain group and set of beliefs - this now empowers them all as citizen moderators.
2. It further empowers this group by giving them the ability to remove other members of the group's ability to moderate comments.
3. It increases the "cost" of commenting far more than most other moderating proposals would. Not commenting on a popular post? Why bother. Continuing a conversation in replies? Again, why bother.
4. It had such a poor specification that cperciva found a critical flaw in the implementation details in mere minutes. If pending comments is an answer to a problem, then it was not the sort of answer that would have been approved by this comment system.