I'm slightly annoyed that Brood Lords are 4 suppply. I hope Blizzard doesn't nerf them too much before the game is released. For 4 supply, they better kick some Terran ace.
As far as I'm concerned, Terran have BC, Toss have Carriers, and zerg FINALLY have the BL, at the cost of guards. Seems even to me. Teching to BLs is a total pain in the @ss. They have been nerfed, they used to have 275HP, deal 25 damage, AND have +2 armor to start. they cost like 4 supply, and are slow as hell. They are seriously awesome, boerderline broken...but they aren't. I love em =)
As far as getting past tanks? For starters, dont. Let em turtle and take over the map, then get broods. Or, try nydus?
Sometimes if you are forced into a defensive position, it's hard to prevent them from taking over the map. You're assuming that the Zerg player has the upper hand and is winning in every game. I thought the Nydus was nerfed and takes a hella long time to spawn now?