It doesn't have to be "downvote points", perhaps it's a limit on downvotes per day, perhaps it's a hit on karma (every downvote means a costs x number of karma

It doesn't have to be "downvote points", perhaps it's a limit on downvotes per day, perhaps it's a hit on karma (every downvote means a costs x number of karma points, and below a certain karma you can no longer downvote), etc. There's always going to be a way for determined users to get around the limits, but if it can reduce the number of reflexive downvotes from most users it could still be a net win.