Sejuani had a 57% win rate in all ELOs despite a 300% increase in pick rate. This suggests that her recent indirect buffs has pushed her to a state where not only is she good, new players can also play her well. The most obvious conclusion is that she is overpowered and should be nerfed.
Even so, the nerfs Sej will be receiving will only weed out the FOTM players looking for freelo from the ones that actually knows how to play her.
Lissandra's Q received a 30 damage deduction: that is hardly any at all. Nonetheless, it wasn't as if she wasn't overpowered to begin with. Despite being a mage, she is excellent against assassins, and can hold her own against other mages.
Lee Sin actually takes some practice to play proficiently. I won't deny that he is OP, but he is no where near as good LoL forums makes him out to be.
Zed is also easily shut down by a number of mid lane champions. Annie, Malzahar, and LeBlanc can easily obliterate him. He is only a problem when his opponents go for squishy picks such as Orianna.
Against another melee carry, Zed doesn't stand a chance and can bring nothing else to the table besides damage.