Depends if the contents are self-generated or community-driven. If they are self-generated, some people are suspcious that ******** outbound links may drain their link juice. This is partially true. But like everything, just nofollowing every single outbound links for Google, is an obvious symbol of über-optimization.

If they are community-drive, nofollow is an standard... you cannot trust which type of links your community is creating unless you have a huge moderation platform.