Or, if the purpose is to have high karma users function as semi-moderators, why not just keep the upvote/downvote system but make a comment's precedence a function of weighted upvotes based on the karma of those voters? I guess that still wouldn't prevent a very deep off-topic tangent off of an ancestor post that's high up on the page, but it seems like a simpler and safer solution with much less potential downside, and there are always specific solutions to the above problem such as collapsable threads.