Guys, I found new broken thing
Or rather, was defeated by it.
Fervor of Battle mastery + champions with multi-hit abilities, notably Renekton (and I am quite sure Shyvana will be just as ridiculous).
To give you some insight on this - at level 3, Renekton E, AA, W (rage) combo deals additional 44 damage from Fervor of Battle - with no stacks beforehand (and he ends up with 10 stacks to pummel you for additional 21 damage with every next AA). For comparison: Thunderlord's Decree will deal 30-36 damage at this level, while having 20 seconds cooldown.
I'm calling it now - this mastery will be broken for multiple melee champions and will be nerfed (or they will nerf champs first and then the mastery - because that's how Rito does things).