It really brings home to Donna – and to us – that nothing is ever black and white and that everything is a tradeoff. The bigger picture.

Thank you for this bit - it makes it so much clearer for me why she spends so much of the episode not getting it, cos I came to the episode with that understanding even the first time and it had honestly never occurred to me that she wouldn't have it...

But then I thought about why I understand it, and a large part of it is being a Trekker and general SF fan from way back so I sort of have that idea burned into me already, that you (usually) can't save everyone and you have to choose. And while we know that Rose and Mickey have at least some SF background and have been exposed to this sort of trope too, and Martha seems like she might have as well - well, what are we told Donna's into? Reality TV and celebrity gossip and all the shallow sides of the media and that seems to be it. I'm not calling her shallow herself, I'm just thinking that maybe she's not primed for this. So the deep end is deeper for her than it otherwise might be.

Feel free to pick that apart, obviously, I'm just speculating here.