Also, as Ann points out: spoilers. Big, big SPOILERS. I do hope I didn't ruin it for anyone - I'm so used to being the last one into a particular fandom, it honestly doesn't occur to me that others might not have already seen it.
There's something I can only describe as "Fantastic!" about seeing this so soon after the fiftieth anniversary special. Because this is, as we've recently discovered through that special, the man who's quite recently triggered a genocide of his own people in order to save the universe. He's ready to do it again for the greater good, but there's this act of faith on his part that just this once he won't have to. Just this once, everybody gets to live.
And there's such pure, unadulterated joy in that. Like I said: fantastic.
I think what I find so encouraging about this is in our times, in American culture particularly today, I see a good deal of protecting, of fearing, of being worn down because even when we are willing to do the right thing there's always a cost going along with it. It's a cold, cynical age in many ways, and one where if you see the human costs of our actions, it's either wearying because they seem so overwhelming or else you have to grow calluses over your feelies. Which I guess is nothing new, really; it just seems like sometimes the collateral damage, the people who fall down so the status quo can be maintained are alternatingly overwhelmingly huge and so small as not to matter.
And then we get the Doctor, begging for just this one day where everyone lives. Reaching out, physically touching the infected child (which yeah, may not be a risk to a non-human but in the moment it feels like one if you know the episode). And he gets his wish. Everybody lives.
That's worth a smile as wide as his, I'd say.