Lethality is broken; The initial backlash was correct
Having played many times with/without lethality on a champion that is hybrid damage with heavy leaning on MAGIC DAMAGE... I find lethality to be better than any of the alternatives. This is on 
Lethality itself might not be broken if it were taken in isolation, but lethality items come with fringe benefits alongside fairly high AD, the combination of which means a character is very mobile and also highly protected from burst. This means the only remaining option is to fight head-on which is generally a bad idea because with mobility comes the advantage of choice; You engage when is advantageous for you and disadvantageous for the enemy.
This is on top of giving the best raw damage boost available. So either nerf Lethality or nerf the MASSIVE benefits that are tacked on to lethality items. That is all.