This has the potential to radically change the game! It'll be interesting to see what defines a soft wall. Shapes, like plates and sheets? Materials, like dirt and wood? Or is it dynamic, based on the current HP of the block?
Scenario: While mining, you hear zombies digging into the dirt above you. Instead of having to surface or dig your way to them, you listen carefully and shoot them through the dirt.
Scenario: You're in PvP, trying to get a guy stubbornly holed up in his hardened bunker. You damage one concrete block enough to make it 'soft', but you don't destroy it. You train your sniper rifle on the block, and when you see the cracks start to disappear, you fire. The poor guy on the other side, trying to make repairs, is dead before he hits the ground.