Opt in would require opt out as well. And that has the potential to be exploited, I would think.
Everquest had several PVP servers once. And each non-PVP server had a number of NPC's (Priest of Discord) that could flag you, permanently, for PVP. In playing EQ since December of 2000 I only ever ran into one person with a red name (signifying PVP enabled) out of the thousands I've seen there. It just was not a popular thing to do there, and I doubt it will be all that popular a thing to do here.
It would be interesting to see what percentage of the population has more than, say, 10 or so PVP instances under their belts. 10-20, or more, would suggest that the person likes to PVP. Fewer would mean that a person tried it and did not care for it. Though there are those who don't care for it but did it for the glamour or the challenge log ...