So you have a group of trolls that all upvote each other's comments. Unless the system also penalizes upvotes then they get disproportionally too much karma. And if the system does penalize upvotes then somebody that always posts really great posts gets too little visibility because his upvotes look trollish (ie maybe they come from the few people that really recognize an expert in a small domain). It seems that an automated system is always going to promote mediocrity.