Alternate Character Interpretation: The 456: really Extra Sufficiently Advanced Aliens, or just bluffing? They clearly do have advanced virology, but we never see any other technology, and the only physical defense they rely on was mundane bulletproof glass. Making all the children of Earth speak in unison isn't much more impressive than what the Sycorax did, and they were bluffing. This ambiguity only makes the story even more tragic, but left some viewers wondering if UNIT or the British government had really tried everything before surrendering.
Prime Minister Green, when he orders Frobisher, his most loyal ally, to sacrifice his own children to the 456.
The ministers of the government choosing to protect their own children no matter what and then taken even further when they all agree to sacrifice the poor and disadvantaged children to save the respectable middle class ones.
Ruined FOREVER: For many fans was immediately invoked upon the ending of "Day Four", and even more for Day Five, which ignores the heroic momentum of the last two parts in favour of being dark and bleak.
This in itself has the same Broken Base reactions to Ianto's death, about whether or not the fans want another series and/or will bother watching it.