Down voting on Stackexchange is a privelege as well as losing a reputation point when you downvote. Down voting on Reddit is controversial and alienates a lot of people from using Reddit. https://www.reddit.com/r/TheoryOfReddit/comments/36rb4r/if_n...

StackExchange it costs you to downvote someone.

On Reddit, downvoting just encourages group-think as you will get downvoted for voicing an unpopular opinion.

Popularity and constructivity are two entirely different things. As mentioned by another post, StackExchange introduces a 'counter-weight' - Reddit is widely perceived as "not constructive at all", and is thus not relevant to my argument.