The state of Crit in league is a big part of why item diversity sucks ass.
I'm not saying "Oh Crit is so weak that you never see these poor items picked up". I'm not saying "Oh Crit is so strong that they're literally the only items getting picked up". I'm not saying anything about whether crit is too strong or not.
I'd like to make a comparison between crit in League and crit in Dota.
In Dota, Crit is a subset of abilities. Lots of heroes have them innately in their kits, so they don't need to buy an item to let them crit. If you don't have crits but want them, you spend a bunch of gold on an item that gives it to you.
An item. As in, you buy one item, taking up one inventory slot. You can't spec your entire build into crits, since there are only two complete items that give crit and they roll seperately. You can dip into crits and you can still build basically whatever the hell you want otherwise.
Meanwhile, look at League.
If you're a carry, and you want to use Crit, you need to invest three item slots into crit.
to make your crits do anything
Something out of
to give you attack speed to trigger crits more often
Something out of
or
to make your crit a reliable DPS tool
After that, you only have three slots to work with, so you just need to slot in what you need.
because your champion probably doesn't have another option.
Some lifesteal item because you need it to survive.
or
or 
So yeah.
IMO, Crit in its current state is bad for the game.
It's not because "RNG BAD" or "BUFF/NERF ADC PLZ", it's because every crit build is the goddamn same, and until crit gets completely overhauled there's no way of changing this.
Also, this is neither here nor there, but if crit were removed from their item line, the Zeal items could be made not only applicable to the entire roster, but just much more interesting in general.
and then needs to flush out the rest of his build with other stuff.
This would limit ADC's raw, unmitigated DPS to something more manageable (you know, like in the seasons before season 6) and allow for more build diversity.
?
Hardcapping Crit Items at 2 would allow a retooling of this item away from Crit Chance. Maybe turn it back into a pure AS item? Or give another interesting stat line?
and Lethality mutually exclusive. As in, purchasing Cleaver hard sets your Lethality to 0.
"wtf Malphite is OP he literally takes no damage"
"What? 0/3 how are you not dealing damage to hi-"
"Tryndamere has up to a 35-50% chance (based on level) to crit for 2x damage, based on his current fury".
Removed doubling crit chances.
"Yasuo has a 25/50/75/100% chance to crit for 1.8x damage at levels 1/6/11/16".
Removed crit chance ratios. AD ratios on these abilities now additionally scale with level. Harrier CD lowered based on level.
Removed crit damage ratio on E. Multiplier increased by 5%.
Removed crit chance > AD conversion.
AS > AD conversion increased from 25% to 44%.
people bitched and moaned so they changed it to melee only, nevermind that this actually meant that the ADC dealt less damage overall and that there was less damage in the game overall, you know, like the boards always say they want. When ADCs could build
should exist, since both of them fill the exact same goddamn niche.
and 
maybe she is the only ADC doesn't require crit items besides
and both champions have amazing pick rate compare to other adc champions.