Did anyone else notice the motif of "eyes" used throughout the series? It's pretty obvious after seeing the intro to every episode (which has at least three zooms into an eye, associating it with the scar on John's chest). Plus his friend paints eyes all over his apartment...(alluding to Escher's "Eye")...and plus they're always referencing the eye, like when John says something like "I look into other eyes and I see whether or not they're bad, but I look into my own and I see nothing." So eyes are associated with identity, the soul, etc...as well as knowledge. AKA the all-seeing eye...and knowing everything...our "third eye" and everything. Yeah.
One thing that took me a while to figure out was the shot from the pilot (shown in the intro to every ep) where you're looking at him from a bird's eye view where he is getting up from the fetal position or whatever, and it looks like the area directly surrounding where he was laying was covered in salt or something. I still don't know what that stuff is, but I get the purpose--it forms an eye. The salt-looking ground is the iris or whatever while John is the pupil, the knowledge, etc. Ahhh...cool, huh?