I have implemented email reminders feature in PTM. Basically it send you an email on a daily basis if you have past deadlines.
While implementing this feature, I had to choose between default opt-in or opt-out for existing members. Well... I have chosen opt-in, I couldn't resist :) But that means that maybe a poor guy will receive a email per day without maybe wanted that. It is easy to opt-out through. But still.
What should we do when implementing a new feature that is critical, that we want to advertise, without choosing to subscribe all users by default ?