I’m not going to go through every single mastery change--that is easy enough to see here. However, the big picture takes a bit more time to figure out, so I’m going to cover that so you know how the game has changed. I’m going to cover it tree by tree, then cover an overall view and review the “garbage” points (which, thankfully, are fewer than Season 3’s).
Tanks that dip in Offense deal less damage. Offense is a less friendly tree for tanks. This is the first and biggest change to understand. Jungle Amumu and Sejuani had 8% penetration. Top bruisers that went 9/21/0 had 8% penetration. Those masteries were replaced with a flat 5 damage and 8 AP, which are nicer for early game action (especially on ad tops), but the longer the game gets, the worse a non-damage-building-21-defense champion will have it as far as dealing damage goes.
Offense is nicer for dipping for cooldown, as Sorcery now gives 5% instead of 4%.
In fact, there is less free penetration overall and for carries, the offense tree simply isn’t as good. Instead of 8% penetration, it’s 6%, and there are no flat points in addition. Here’s an old Season 3 calculator. Put 21 points in Season 3 and 21 in Season 4’s. And no, Warlord with 6% armor pen is not better than 5 armor pen and 8% armor pen. If you get 200 bonus AD (that’s when you have almost 300 base), that’s an extra 10 damage. 2% / 5 flat penetration is worth more than that.