1. Opt-In vs. Opt-Out vs. Tough Luck
This is a tough one. Clearly Opt-In is safest, but would it be such an impediment that it would kill the feature before it can even get going? Opt-Out seems to be a minimum requirement and it is the technique that #FollowPacks has adopted. Tough Luck should simple not be OK.
Would suggest when a pack is published, and again in any directory, users should be advised that they can ask to be removed and given a clear means to request removal.
4/n