Time

I think a lot of people and teams under estimate the value of "time" in LoL. By time I mean, the amount of time it takes to kill XYZ (people, turrets, champions, waves, buffs etc). There's always a value of each objective/thing you're trying to do and you need to be able to understand which one is "worth it". 

As an example, usually the amount of time needed to kill dragon/rift herald is too high relative to gaining experience or even backing. Say, Rift herald takes 15-20 seconds to do, and during it your top laner misses out on a wave/half of exp and lane control, it just isn't worth it. 

Say you have 20 seconds on enemy respawn timers, and it takes you 15 seconds to kill an inhibitor, or you could do baron which takes 30 seconds to do (to travel to, etc). You have to be able to figure out which one is more time efficient and more value for said time while keeping track of the general way of how to win the game with your team comp (ie do you siege or do you teamfight?).

Another big thing people fail to grasp is when someone is split pushing, and you need to send multiple people to kill him. The guy that is dying can buy time by running away from X objective or surviving as long as possible to delay enemy resource. For example, say I'm level 3 in their forest and I'm going to die, if 3 people are chasing me and I waste 10s of each person time that they could be in the lane, one of them is going to get screwed for it. So, although I died, it might be worth it (in the case I got a kill or stole a camp or gave a laner an advantage because he didn't roam in). 

Every second in LoL counts, it's important to know how long everything takes to do.