I'll start with the latter question: it depends which link you give to your friends: simply click 'Link to this wall', and you will be presented with a list of permissions. For each set of permissions you get separate link. It simply depends which link you give to your friend (it's more of an social thing than technical, I think).

As for threads/replies: yes, this is obvious thing to do, but I wanted to check if it's really that useful and demanded before actually implementing this. The same (possibly to lesser extent) goes for voting.