Absolutely! Open World PvP is what keeps the game interesting and the world feeling alive. I do think that some restrictions need to be put in place to hinder griefing. This can vary from level restrictions (i.e. can't attack people who are more than 10 levels below you), escalating PvP zones (see Albion Online), karma system etc.
I do believe that Ashes of Creation is planning to implement a perfect open world PvP system. There will be unrestricted open world PvP which is subject to their corruption system (karma system) and this corruption is meant to hinder griefing. The great thing about their open world PvP system is that there will be many ways to engage in open world PvP without having to worry about the corruption (karma). They accomplish this through their caravans (PvP areas around a transport unit), guild wars and node wars (faction wars).
In short you can attack anyone you want, but be subject to corruption or you can participate in one of the other open world PvP options that do not incur corruption. I definitely recommend that you do a little research on their open world PvP systems.
I personally have no interest in MMORPGs that do not have open world PvP and I will not play them.
There must be restrictions on who can attack who. We are thinking of a bounty hunting system where if you attack too many lower-level players you get flagged for bounty hunting and get a reward for your head, so everyone would be able to kill you and get a reward of some sort.