Look at the post game graphs. All of those are technically factors in the game overall. It includes damage in general, and specifically dealt to champs/structures, objective participation, kill participation, vision score, creep score, healing/shielding to champions, deaths, and damage taken.
also, you can look at the stats of champions you play somewhere in your profile tab (i'm not at home or I'd know the directions to see it) that compare some of your stats to the avg in each division. I think that average has something to do with it too - if your score is far above the average of your elo, you'll get your S. If it's about on par with it, I think it averages out to about a B. If it's below, you get a C or lower.
This is super late...but in the future, it should help.