It's been a while since I've been on slashdot, but back then their system was quite sophisticated. You could not always vote on stories, only when temporary moderator status was assigned to you by a random process. Additionally, sometimes you would be assigned meta-moderator status and get to vote on the quality of moderators ratings. Those meta-moderations would in turn influence the likelihood of the rated moderators to become moderators again in the future. Also, they have categories for the votes ("funny", "insightful", "troll", ...).