This is a comment from alyaza, one of the mods:

vote brigading is one part of it, but another influence of this decision comes from Tildes, where the emphasis is on quality of discussion and the site accordingly has feedback mechanisms to reflect this–most prominently in lacking a downvote button. (although i should also note we’re going for a more laid-back attitude than Tildes has.) while i’m sure there’s a theoretical way to minimize their impact while maintaining their function, downvotes can easily be used to artificially sway opinion, punish unpopular opinions, etc, and their utility is actually somewhat minimal as a website feature and community control mechanism.

the going theory in removing them therefore is that to express disagreement, you’ll have to at least put some thought into why you disagree with a post, and ideally that will be expressed in the form of a comment which can be used as a further jumping off point for conversation and dialogue. (alternatively, i guess, you could also just accept a disagreement as not necessarily worth your or another person’s time, and move on.)