ok so this thread's back again!
it's true to say that we shouldn't judge this game just by its graphics, obviously there is a lot going off underneath, but i feel you are not thinking outside of the box enough. We don't need to create a "port" of Terraria. We only need to create the illusion of Terraria, or an acceptable simplification of it. There is no need to store the entire map in RAM, for instance, since only a small portion of it will have been directly modified by the user at any time, and anything that happens a long way away can be done much more crudely, and less often, deferring details until the player actually goes there.