First, apologies for thread necromancy.
Second, I hope I'm not the only one who's been playing the demo for Side Second, hoping against hope that it will magically be complete instantly...
Third, I felt compelled to say something about the original:
I think that even the (IMHO, true) interpretation offered by Eclipse about the human face on the statistic is still not the "message" that moves the audience so much. The way I've read it is that the transformation of the faceless statistic is
used as a tool to further the real point of the story.
Think about it: in the beginning, what is Setsumi but the faceless statistic? She sits, perfectly apathetic, completely detached from the world around her. But then along comes the protagonist and the theme (or, properly, themes within a larger message) is introduced. First, you have the idea of the acceptance of the inevitable. The characters are going to die and they both know it and accept it. But then there's the idea of living life anyway: carpe horem while you still have an hour to seize. Why do you (protag) go to Awajishima?
Why not? It gives their lives a purpose, a goal, a meaning.
On the seventh-floor ward, their entire purpose was to march complacently and stupidly to their deaths, controlled and contained, like proverbial lambs to the near-literal slaughter. But when the protagonist takes his father's car, it is a profound "no" in response to the containment of the hospital - and a profound "yes" to himself. His driver's license was one of the things that was (at least somewhat) unique about him, it was something he could do. On the ward, all he could do was shut up and die, but by leaving he could take his own life into his own hands.
So too for poor Setsumi. With her time coming near, she slips further and further into the "faceless statistic" of people dying from disease. She denies all of the things that make her unique (her knowledge of botany and maps, for instance, or her desire to die in a manner of her own) in an attempt to follow along with easy, anesthetized death. So when she goes with the protagonist, when she tries on the new clothes, when she gets her driver's license, when she models in her "bikini", and when she at last walks out into the sea to die, she too is saying "no" to the attempted dictation of others as to how she should live and die.
While this message may seem somewhat nihilistic, it doesn't have to be: the idea is to stop being a mindless sheep and to take control of one's own life. It's short, even if it isn't your third visit to the hospice ward; and it's your own, to do with what you will. You can live it out the way someone else tells you to (become yet another faceless, nameless death on the Hospice ward). Or you can find your own purpose for living life, and live that life according to that, your own principle, your own imperative, if you will.
That is what, in my humble opinion, the story is really
about: Setsumi turning away from that mindless, faceless death in the lifeless hospice ward (perhaps this is the reason for the negative exaggeration of the hospice's portrayal?) and riding off into the sunset with the protagonist. In this way, the story is wonderfully joyful, as you begin to see Setsumi rediscover her own uniqueness, her own identity, her own purpose. However, the story is made more effective by its compression of time, which itself is done effectively with the use of terminal illness. To make the story manageable and understandable, the time frame is reduced, and what might be the entire life of anyone else is summarized in a few days, and the suicide at the end is the capstone: instead of a pessimistic, nihilistic expression of dying because of a lack of purpose, Setsumi's suicide is the ultimate expression of her own, in a sense, redemption. She has taken back control of her life, taken back a meaning for it all, and she gives the final, forceful finger to the "mindless sheep" who coast through life on the shoulders and ideas of others by taking control even over her own death. She has so much control over her own life that she chooses how she dies. Can we not each take this metaphorically? Can we not each live so that when we die, we have no regrets about the past, only regrets about the lost future?
In this way the story is also sad, for it is a simple, succinct, and moving expression of the fact that to be human is to be a character in what is ultimately a tragedy, in that everyone dies in the end

Tolkien said of Beowulf "He was a man, and that was tragedy enough for him, and for many." Death is always sad, and the overwhelming presence of it in Narcissu gives it the illusion of a sad story. However, in the end we see that Setsumi wins with a royal flush over the real tragedy: not living what life she had. She learned that she didn't have to stop with mere acceptance of the eventual end, that there is always the potential to go further, to realize that, since it's going to end, there's not time to lose!
That's why, in the end, I cried, not only because she died, but also because her death was truly beautiful in her utter victory over the apathetic life.
But I'm probably just reading too far into it anyway ^^;;;