WTF is new Rammus W?

Because you get a higher ratio of your total defenses as a bonus with rank?

His W gets a ton of resistances. Lets say with old Rammus you had 150 armor or MR, with his old W you'd have 270. With the new W at rank 5 (20 fat +70% total resistances) you will have 289.

So now all it does is up your tank stats, but that number can get bigger now after a certain threshold.

I always maxed his W last anyways, so I don't really care about this change much. The big thing with the W though is comboing it with your E. When you taunt someone then use your W, the entire 6 seconds of your W you will have the attack speed bonus from your E + the damage from your passive gets that +50% boost. The interactions with his other spells is the highlight of it now.