The only thing that was decreed as regards to Mary by the first Seven Councils was that she is the One who Bore God (Theotokos), in response to Arian heresy claiming Christ to be a creature of God, the Son lesser to the Father, and in response to those heresies that called her Christotokos.
As for her death and assumption, the Orthodox always believed that she died and was assumed bodily into heaven on the third day of her passing. Many Catholics believed it as well, but the Latin Church was silent as to her death and assumption which would bring up other issues if she never died. (I guess we could say the same thing about Elijah, but he is supposed to come back from heaven and then die!)
I believe that the Catholic Church may still not teach specifically whether Mary did die. The reason for this is of course the Augustinian concept of the original sin which the Orthodox recognize but not as is understood in the west. If she had been cleansed of all sin at her own conception, and lived a sinless life (by choice, as she was 'equipped' to do so by special Grace), then she would have no reason to die any more than Eve would not have died had she not sinned.
St. John Chrysostom theorized that Mary could have sinned at the wedding in Cana, which would be enough to condemn her to die, but at the same time then it would be contrary to his own liturgical elements praising her as "pure."
I am not aware of any other father theorizing about her death and assumption. The only thing regarding consensus patrum one can say is that the East believed she died and was assumed bodily to heaven all along.
You anticipated the follow up very well, thanks. I suppose I've never associated original sin with physical death so I wasn't thinking along those lines. What is the thinking on what physically happened to her body when it was assumed? Was she put back into it and still "using" it? :) I don't understand the purpose for her assumption.