Ok,so I've started a new single player world and set it on Hard difficulty,but,zombies don't break doors,mobs spawn rates are very low and they do act like it is not on Hard.Their tracking ability seems to be affected also.After you kill a zombie,maybe another 2 will come for you but not more than that,they dont come in hordes anymore.
P.S Sorry for my english.
Zombies now only have a chance of breaking doors, it is not 100% (per Dinnerbone). Spawn rates and the "horde", as well as the AI is being rebalanced in the current snapshots so none of this is a technical bug.