I really liked Conquer Online, mainly due to its open PVP system. Some guy just picked up a meteor? PK his ass and hope it drops when he dies. Of course that works both ways, so you've gotta be quick on your feet to run away to the town before someone gets the idea to kill you for your inventory.
There's also an AFK leveling system which I thought was cool, you can get some free levels while you sleep.
There are limits to the PK thing though. One you first attack an "innocent" person, your name will begin flashing. This means anyone can attack you without fear of a penalty. It also means that while your name is flashing, the guards will attack you. Kill too many people unprovoked and your name will turn red. This means you're fair game to be killed by anyone without penalty. Kill even more people and your name will turn black. Now guards will attack you on sight and if you die, there's a chance you'll drop your equipped items.