@julian Potentially, but it's open for abuse. Wouldn't take long for someone to create X accounts and flag the same comment X times, or get a group to do it. Users can flag posts, so spam is pretty much already covered. Registered User moderating very rarely works in most cases. Even places like Reddit or Stack Overflow just push that content to the bottom. So it's not seen.
NSFW tagging is down to the forum admin, they either allow it, or they don't.
A possible solution could be adding reasons behind why users are flagging content via a radio button lightbox, so rather than just sending it to a moderator/admin, admins could specify what happens when a post is flagged as spam, or NSFW. Or if no rule is created, send it to an admin. So IF topic flagged as NSFW, add tag NSFW and send it to mod for confirmation.
Basically IFTTT, NodeBB edition. 