I think that the most significant element of the last line brings to light how Eliezer is incapable of recognizing his own reflection.  On a physical level, it brings light to the significant manner in which Eliezer has changed over his experience of surviving the ordeal outlined.  On a more psychological level, the inability to recognize himself brings to light how much Eliezer has changed.  The ideas and hopes, promises and possibilities, in which Eliezer believed are now long gone.  At the start of the book, Eliezer was someone who believed in God, hoping to follow Moshe's advice of gaining the courage to ask God the questions that plague his being.  He was a boy who believed in his community and in his family, a boy who possessed a healthy respect for his father and the role he played in the community as a leader.  By the end of the narrative, Eliezer has effectively renounced his spiritual side, lost connection with all others, and sought only his own survival, akin to an animal.  He no longer viewed his father with the type of reverence that was seen at the start of the narrative, rebuking his father at different points, and not responding to his cries of suffering.  The last moments Eliezer has of his father is of his wailing in pain, and Eliezer almost secretly hoping that he would "go away."  When Eliezer cannot recognize his reflection, it is a significant instant where he has changed so much that he cannot fully embrace or appropriate who he now is from what he once was.