For me, it's a combination of both the language, which I think is stunning, and the sense that I have at the end of it that the questions it is trying to answer are mysteries that we can only see as shadows.

It starts by having the wrong guard (the one taking over, not the one who's still standing watch) asking "Who's there?" and it ends by telling us that we're all there, really--in some way, we're going to be confront by questions about our lives that are as big as Hamlet's, and we can only hope that we'll handle them at all.