Energy: The "proxi"-ressource
Energy has become a ressource with the sole purpose of justifing powerful low CD spells - "b..b...but they cost energy!" sounds much more impactful than "they're free" . Looking at Zed and Lee (lane). Energy is close to no restirction at at, it regenerates incredibly fast an in the worst case scenario it delays a cooldown by or 2 seconds. At the end of the day it might as well not be there.
I have to admit, that this is not completely true for Shen or Kennens mobility skill which are quite expensive - on the opposed end we however have champions like Lee Sin and Zed who might as well be ressourceless.
I feels like energy - or energy costs for certain skills - really have to be adjusted at the current state it's neither balanced between energy based champions, nor balanced compared to mana or health users.