Say someone’s just walking down the road; you don’t know him and he doesn’t know you. Can run up to him and attack him?

Depending on your level, I would say yes or no. If you aren’t the same level, a warning box will pop up that says, “Be careful, you’re probably not strong enough to defeat this guy”, something like that. You can do it at your own risk, but in general you will have to join a group if you want to be safe. You will still be able to attack by yourself if you are in a group, but in general when you are in a group many people will be able to help you defeat other players.