I support world PVP.
The only problem that I see could that could arise is a high level player camping a low level player. Personally I could care less if someone were to camp my low level character, but you will get those who will cry about it. To be fair there should be a range of levels which a high level player can initiate combat with.
For instance a level 50 could only initiate combat with anyone levels 45-50. I guess it should be a +/- 5 level difference for anyone. This would keep people who are trying to quest their low level toons from getting camped all day.
I also think that anyone outside you level range can initiate combat with you. Shouldn't be too hard for you to take down a lower level player even if they get that jump on you.
Then there should be areas where anyone can engage anyone regardless. I guess we could start with the PVP areas of Ilum and Tat.
I don't know if any of you ever played Runescape but it's pretty much the same model, and it was good for keeping fights even.