[shagie]

Down votes on questions are free. Down votes on answers cost 1 rep.

You need 125 rep to be able to cast a down vote (votes for people and anonymous users with less than 125 are recorded in an anonymous votes table ( for example, http://data.stackexchange.com/stackoverflow/query/209365/ano... ).

Hacker News has down votes too... and you can't down vote until you get a certain amount of karma.

The rep lost from down votes are an investment in site quality that you get back if the post is deleted.