To be honest, it depends on how busy of a news day it is. However, breaking news should be new, should be important, should be something of interest to a significant number of freepers, should not be analysis or opinion, and something that isn't just a slight incremental update of an ongoing live event (live threads can be BN, and can have the title updated by polite requests).
On busy news days, the threshold for what can be in BN goes up. On slow days, it goes down.