This has the second showing (if you look carefully) of the special weapons Dalek in the background which was only ever seen in Remembrance of the Daleks - 4 of 4 which has a single 'cannon' weapon and no domed head.
The presence of Jenna Coleman in this episode was successfully kept secret, despite the episode having several preview showings prior to broadcast. After broadcast, Coleman and Steven Moffat both issued statements thanking fans and the media for keeping Coleman's debut - months before her official first episode, the 2012 Christmas special - a secret.
Oswin was originally conceived as being a fan of Elvis Presley, whose music would play throughout the episode and inspire the Doctor to give her the nickname "Blue Suede Shoes". However, when obtaining the rights to use these songs proved problematic, it was decided that the soundtrack to Bizet's Carmen would feature instead.
Rory was originally meant to have a beard in this episode, which Amy would hate and he would shave off at the story's conclusion. Between seasons, Arthur Darvill was appearing on stage as Mephistopheles in Doctor Faustus, and Steven Moffat planned to have him keep the beard grown for that role. However, this plan had to be abandoned when it was decided to make this as part of the second recording block alongside "The Angels Take Manhattan".
The trivia items below may give away important plot points.
Oswin is played by Jenna Coleman. Coleman was originally cast as the Doctor's new companion and scheduled to make her first appearance during the Christmas special. Steven Moffat came up with the idea to cast Colemen in this episode in order to generate speculation about her character. The Christmas special later revealed that the new companion's name is "Clara Oswin Oswald" and that she is somehow connected to Oswin.
Steven Moffat realised that the Doctor would never actually see Oswin in her human form. This meant that the actress portraying Oswin and the actress cast as the new companion could be one and the same. As such, Moffat did away with a suggestion that Oswin might be a Dalek whose insanity has caused it to believe itself to be human.
Although stressing that the first syllable of "Exterminate!" sounds like eggs was new to television, it was not the first time the notion appeared in licensed Doctor Who. It was done about 20 years earlier by Paul Cornell in Metamorphosis. The usage is remarkably the same in that Doctor Who Yearbook comic, except that it's the Seventh Doctor who is turning into a Dalek and saying "eggs".