I'm curious about how to counter mass BCs as well. In Starcraft 1, massing capital ships wasn't too bad since the same minerals/gas pumped into Goliaths/Hydralisks/Dragoons can take on mass Carriers or BCs. Starcraft 2 seemed to be different for my one encounter with them.
I played against a ridiculously turtling Terran yesterday. He hunkered down with Tanks, Missile Turrets, and Supply Depot walls. He also had a medium-sized squad of Vikings that prevented any attempt at drops. I decided to counter this by expanding all over the map. I maxed out with an army of Zealots, Stalkers, Sentries, and High Templar. I decided to wait for him to move out, since I wasn't feeling confident charging head on into tanks. He went for mass BCs and moved out with those, supported by 10 tanks. I sent my army in, and I was able to kill all his tanks and half his BCs, but at the cost of my entire army. Even with Guardian Shield and mass Psi Storm, I still lost the battle. I was able to win the game since I could quickly replenish my army with my mass expansions, but losing all my units to the BCs was surprising.
Anyways, is there a hard counter to mass BCs? My Storms didn't seem to be super effective since BCs have so much hp. Would Feedback work better? Is the Feedback damage calculated in a 1:1 ratio between mana and damage?