The biggest problem with "direct attack other players PVP" as you call it, is that no matter how it is set up there will always be players who grief other players for the simple reason they find it amusing to upset other people.
A 12-hr trade blockade? You keep losing to other players repeatedly (because those players who like to grief other players WILL keep attacking you, and will co-ordinate with other like-minded people) and suddenly that 12-hr trade blockade becomes permanent!
Every single other game out there that has PVP, has it set up so that you can cause harm to other players, and in every single case you have a LOT of players griefing the weak simply because they can and because they enjoy causing harm to others.
Wouldn't it be nice if we could help each other out instead of hurting each other? Why do you have to bring this "i'm the best and I will crush you into the dirt" mentality into every game?