When I lived in Colombia, I encountered a culture that was more acutely aware than the one I grew up in of what it means to turn one's back on another person. Perhaps it wasn't a huge deal, but people seemed nonetheless more aware in conversation of who was at their back and how that might be perceived as impolite.
When God does this in the Bible, of course, the implication goes far beyond discourtesy. For God to turn away from a person or a people is to ignore them at best, or to be angry with them at worst.
And so, if God's back is unfavorable, then the face of God is a metaphor for God's favor. Turned toward us, favor is given. But turned away from us, it is withheld. If I think of God as my salvation, then certainly, the face of God is as the sun: warm, cheerful, life-giving. When it faces me, my shadows are behind me. But when it is behind me, I must walk into the shadows I myself cast.
Dear God, let your face shine upon your servant; save me in your steadfast love; in his Name who taught me to pray: Our Father...