ADC (un)balance
I could say many things, but i'd rather just post a chart
http://www.leagueofgraphs.com/champions/stats/adc/diamond
2 champs alone have 101% pickrate and they both have positive winrate, Then you have Corki and Kalista being decent because they are both busted (Kalista is inherently busted, Corki is busted at the moment due to being overtuned), and the rest is complete garbage, like, absolutely terrible garbage.
Sivir constantly jumps from 48 to 50 due to her being played by 2 players total, Jinx is basically the same but her winrate is a lil better due to her kit being stronger in soloQ, the rest is almost terrible
This is the case with the ADC role only, the rest is quite ok
http://www.leagueofgraphs.com/champions/stats/middle/diamond
http://www.leagueofgraphs.com/champions/stats/top/diamond
http://www.leagueofgraphs.com/champions/stats/support/diamond
http://www.leagueofgraphs.com/champions/stats/jungle/diamond
Support has got slightly less diversity than the rest, but the ADC role is just terribly stale and incredibly unbalanced.