yeah its crazy how much small stuff you can miss, i pretty much killed all the NPCs you could be friendly with (eileen, adelle, beggar guy) but fortunately it didn't block any of the plat achieves. I think the only really big ones are killing ios too early and not backing up your game before the last 2 bosses.
also crazy to think cainhurst and upper cathedral are completely optional zones, you never see that anymore in Western games. If there is a path in the game the devs feel they have to force you to explore it so they don't get dinged for being too short.
i wonder what the DLC plans for the game are