I think the main problem boils down to:
1. Not doing enough damage, LW was slightly nerfed, PD was nerfed big time, zerkers were nerfed, triforce was nerfed (allthough this only concerns Corki) and Warmogs was buffed. You simply don´t do enough damage early-mid game with auto attacks. Bruisers and junglers with Warmogs just show up in your face and you are not enough of a threat to them to force them off you. Your ability to just 1v1 a bruiser is a lot worse.
2. Not being able to get any good defensive items. Usually ADCs would get double dorans to add a bit of tankiness and then eventually get a QSS/GA, all of these items were nerfed. That 200 hp that you got from dorans blade really helped and you definitly feel squishier. QSS and GA are expensive to get now and they give way lower base stats, not worth it to get at all. So you are basically being forced to either suck it up and pay the price for a QSS or get a Warmogs which you won´t itemize well anyways , you just buy it because it is really fucking OP.
3. Mid/Top/Jungle champion pool changing. No longer will you get a Nautilius/Alistar jungler who works as your second support in teamfights, people play bruiser junglers and that is just another person coming to get you in fights. Mid laners often are not the typical AP carries anymore who will distract the enemies attention from you (Karthus mainly) nor APs who will protect you (Orianna, Anivia). The issue with this is when the mid laner is an AD assasin or bruiser, that is another person coming to get you in teamfights. Not to mention their team as a whole becomes a lot tankier when the mid laner builds warmogs aswell.
So basically you are left with a very small pool of viable carrys. If Riot wants to fix this I suggest they start by buffing PD, it will probablly help a whole lot for the standard auto attack carrys.