Am I the only one who doesn't have a problem when they play Lux? I mean, not all of you here I guess, some of you are defending her kit. Yeah, her E costs a lot of mana and doesn't nuke players who take damage from it, but almost everything else she can do does provide that nuke at a fraction of the mana cost, especially after you get Lich Bane.
Is the complaining because her E is easier to land and you don't like that the easier-to-land spell isn't her highest damage? Is it because you think mana cost should directly relate to damage dealt? Because that's absurd. Her E provides more function than most abilities in the game do, but let me scale down that statement just to make it more palatable: even if you disagree with that initial assumption, you can't disagree that her E provides more functions than any of her other abilities COMBINED. What do her other abilities have going for her? Passive proc, stun, shield, range, and damage. Even if her E did 0 damage on detonation, it's still a ward, has a long duration, is an intimidating obstacle or trap, is a slow, is an aoe, and procs her passive. Let's even completely omit that it can be detonated prematurely, and remember we're already omitting that it does any damage, that's still 6 features compared to the 5 total sum of features from the rest of her skills. Speaking as a Lux player, I say the mana cost and damage on her E is fine: after the laning phase stop using it to do damage, only use it to proc your passive when you can both catch more than two people in it and can R them immediately, and instead start using it as the utility bomb that it is.
Start using it to slow your prey before nailing them with your stun or R.
Start using it as emergency obstacle between walls in the jungle or otherwise when you're being chased (I might not be afraid of the slow, but I don't want a passive+Lich Bane+AA or passive+R to nuke me)
Start using it as an emergency ward when you're out of them or worried the precious seconds you'd spend walking into the jungle would better be spent doing damage to that tower.
It might not be WHY it has a high mana cost, but it still makes sense. To me, at least.