Spam vs free speach. Think there are a few behaviours for what downvotes are a good thing. Tagging in thousands is spam, not free speach and should IMHO be flagged down to zero, and on repeat behaviour, below.
What I do think is that we need a checkbox for safe downvoting. Downvoting cuts in reputation as well as earnings. We should have an option for earnings only downvotes I think. Reserve the deep cutting downvotes for repeat offenders.