I want to be able to assign a single member as the owner, but there are time where I would like to add other members just as observers and allow notification rules based on that (but not confuse them on who owns the card/task). This would be especially helpful as well if the filter would also allow the differentiation.
This is different that adding observers to a list.
Right now, there isn't a way to subscribe other users to a card without adding them as members of that card.
However, if these are members of the board already, you can @ mention them in a comment on the card and suggest that they subscribe to the card. If they subscribe, they'll then receive updates about changes to that card, without having to be a member.
We have some more information about subscribing here: http://help.trello.com/article/799-subscribing-to-cards-lists-and-boards