Before communities and SMT's take centre stage here, the economics should be in great shape to handle it all. The more people come in, the more difficult it will be to handle these issues.
Free downvote is a good idea. It will be abused though and that is a fact. Also even if my down vote of a post costs me nothing, what will it cost the post being downvoted? Can I, without using my mana, affect another person's reward? Is that possible? Also downvotes being free would mean not being attached to say reputation score, so that automatically means all down votes will be equal depending on the gravity of the offense. Oh there will be abuse.
I think the down-votes would be free to a point and not unlimited so down-voting would start to affect mana after a yet to be decided amount of weight. They would still be attached to SP and therefore not equal - same as is the case now. I've not seen any discussion on how rep would be affected - same as now I assume.
There will likely be pockets of abuse.