The whole purpose of lethality was to not make it that snowbally. With the update this patch, it's completely opposite of that. Duskblade deals ridiculous damage making it very easy to snowball with any champ that can use it. It also scales off your own level, which will always be higher than the enemy while snowballing. Before, you were rewarded for fighting the higher level opponents, but now you can just run around and oneshot the enemies that are behind. Also, since the extra damage is on the first autoattack, a larger portion of the damage is impossible to dodge.
I played against a lethality Irelia yesterday and she oneshot me while i had 100+ armor as a midlaner midgame... Irelia isn't even the worst abuser of lethality so just imagine it on Zed, Jayce and Talon right now.