I just hope that due to the game being always online and always multiplayer in nature (in the outer-world at least), there will be a way to stay out of PVP/PK action without compromising the experience. I'm fine with Blizz trying to bring back PK and PVP in a way, but I really never cared or even liked it. In D2, it made it so that I would only play on private games with friends or solo. A playstyle I actually adopted in D3, I've played thousands of hours of D3, I don't think I've played much more than 10 in a public game.
D4 will force me to be playing in a more public settings, as long as PVP isn't shoved down my throat, I'm not too worried. I'd hate to avoid certain side quests or zones simply because PVP is always enabled there.