MeFi already has favorites and flagging. However, flagging is only used to get a moderator's attention, and the final decision on whether to delete something is always made by a human. I think the userbase would be extremely resistant to changing that.

More importantly, an integral part of MeFi's culture is that the moderators aren't just behind the scenes, deleting spam and banning problem users; they try to actively steer discussions away from flamewars/trollbait and towards productive discourse, without being too heavy-handed on the "delete" button. It's similar to what I've seen dang trying to do here over the last few months, but it works a lot better without nested comments.