That's the reason downvoting didn't exist here at first. PG is aware of the problem of using downmodding to convey disagreement (and he's all too familiar with reddit.) But, eventually, he wound up adding it in anyway, to control overall post quality. You can't downmod unless your karma meets a certain threshold, which means you've been in the community long enough to learn not to downmod on disagreement.
As the community grows, however, reaching that low threshold may become easier and easier, so people less involved in the community suddenly get the same power before they know how to properly use it.
To fix this, the threshold should grow with the size of the community. For instance, instead of a static "50-100" points threshold, maybe it should be a dynamic question, such as, "is his karma in the top 70% of the population?"
As the community grows, however, reaching that low threshold may become easier and easier, so people less involved in the community suddenly get the same power before they know how to properly use it.
To fix this, the threshold should grow with the size of the community. For instance, instead of a static "50-100" points threshold, maybe it should be a dynamic question, such as, "is his karma in the top 70% of the population?"