What is the new lethality ?

Lethality gives you scaling armor penetration. It starts out with a base of 60% of the lethality and scales up to 100% at level 18.

Previously the level used for this calculation was the TARGET's level. So if you were level 10 and attacked a level 8 enemy, you would calculate the armor pen from lethality using level 8.

Now it calculates using YOUR level. So with the previous situation you would get armor pen using level 10.

This ends up as a slight buff for solo laners and snow ballers who when they are doing well are often higher level than their opponents. A slight nerf for ADCs since they tend to be slightly lower level due to duo laning.

It also means that lethality will always be giving you a specific amount of armor pen now instead of a variable amount. IIRC you can mouse over the lethality stat now to see precisely how much armor pen you are getting at your current level.

It's better for solo laners now than duo laners so ideally ADCs shouldn't build it and assassins can make full use of it and have a better early to mid game ganking bottom lane.

The issue though is that the damage on Duskblade is currently way too high -- you've got one ADC in particular abusing that and that's Draven.