This is a good question. Do you design a system on the assumption that people are good, or do you design a system on the assumption that people are mean, dishonest, cruel etc?
The best payments processing companies are successful because the mitigate human fraud and dishonesty. Will the most successful online community be the community that mitigates cruelness and meanness, or amplifies kindness and goodness? Instagram is probably doing the best job on this right now…positivity is greater than the negativity. Facebook and Twitter are in danger of drowning in the negativity.
I believe that humans as a species are default good. Be the community you want to see in the world. Click with compassion, be civil and most importantly be kind. If our community is built on those principles then maybe we can design the system based on kindness over cruelty.