The Moff was back in top form for this arc-heavy episode wherein we learn more about River Song than ever before.
A large part of the story arc of the last few series is pieced together and explained. Although there are still a fair amount of questions still to be answered, this story was a very satisfying part of the puzzle explaining the beginnings of River Song and her first real encounter with the Doctor (outside of briefly meeting him as a newborn or as a child imprisoned in an astronaut suit), what made her fascinated with the Doctor, who the regenerating child was and what happened to baby Melody.
The inclusion of Mels(River) as a childhood friend of Amy and Rory and her participation in getting them together as a couple in the first place was a beautiful piece of timey-whimeyness that has become the Moff's signature. And it works wonderfully with just one criticism; I think Mels should have appeared before Let's Kill Hitler so as to establish her as a childhood friend beforehand rather than make the idea seem like an afterthought. Even a brief appearance in The Impossible Astronaut could have established the character as being a part of their lives. That is, unless time is being rewritten and that's the Moff's intent that she was suddenly added to the timeline. We probably won't know that though until The Wedding of River Song is shown.
The performances are great all around, especially Alex Kingston and Nina Toussaint-White as Mels. Not to mention Arthur Darvill who gets a fantastic punch the air moment(or rather, punch the Hitler moment).
Speaking of Hitler; I felt he was underused in the episode as were the Nazis in general. This period of history is ripe for potential stories and would be perfect for a pure historical story without any sci-fi elements aside from the TARDIS and the regulars but its merely utilized as background for the story of River Song. Seems like wasted potential although the brief scenes where Hitler is used as comic relief are brilliant.
The Teselecta is mostly a plot device which is utilized well in the story but it raises the point of now that the Time Lords are gone, it seems like there are an awful lot of time travelers out there doing whatever they want without anyone to police them. Surely this kind of time travel where the Teselecta humans snatch war criminals out of the time stream to torture them is precisely the sort of meddling the Time Lords used to prevent. Good potential for a future story.
One side note; I will say its getting a bit repetitive to kill a member of the TARDIS crew and resurrect them in nearly every story but so far it hasn't affected my enjoyment yet.
All in all though one of the best stories of the series so far and I for one am loving the story arc episodes.
Here's my list of comparing each Doctor's seventeenth stories in my opinion from best to weakest:
1) The Time Meddler by Dennis Spooner
2) The Talons of Weng-Chiang by Robert Holmes
3) Let's Kill Hitler by Steven Moffat
4) The Invasion by Derrick Sherwin and Kit Pedler
5) Frontier in Space by Malcolm Hulke
6) Frontios by Christopher H. Bidmead
7) The Lazarus Experiment by Stephen Greenhorn