Farm is extremely important to get an S.
I've seen some players argue over a game that they should have gotten an S in but don't and I think that some people are over looking the importance of farm. I am certain that a lack of adequate farm is what makes most people close to getting an S because in this game that I played, I had a great KDA and my teammates and I thought that I was going to get an S but I too was wrong. The Tryndamere in that game however got an S and also had a good KDA, but not as good as mine. The difference between us was that he had way more farm than I (I was the Vel'Koz and I got an A+). I will admit, I am salty about not getting an S because one more S is all I need to get mastery level 7 on Vel'Koz. In my defense, i was lacking in farm that particular game because my botlane was struggling really early so I devoted a lot of time to roaming. I feel like I should be an exception because I HAD to roam or else my botlane would have fed to the point of no return without me but then again, I am salty about not getting the one more S that I needed. I hope we all learned about the importance of farming!