Roots: Sam mentions Hagrid (Harry Potter) and Polly Pocket. Farl and Amy's situation is inspired by arranged marriages.
Dialogue Disasters: "That was killer awesome Scotty babe!"
Dialogue Triumphs: "What was that weird noise? A sort of wheezing, groaning?"
"What's the penalty for punching a smug fop on the nose?"
"I've seen things that would freeze your blood. So don't threaten me, don't ever threaten me."
Continuity: The entity is from a universe of fear. It created a bridge from its universe to Phobos via a wormhole. It feeds on fear, but requires euphoria in the mix, which is why it lured the drenies to the resort: pure fear in great enough quantities actually harms it. It is trying to become strong enough to break through completely into this universe, and possesses the body of Eris to facilitate this. The Doctor either kills it or weakens it so much that it can no longer influence Phobos (he isn't sure) by showing it all of his fears, which includes evil from the past and future, as well as things that he fears he may do one day.
Githians are huge furry humanoids. They are forbidden to marry outside their species, since they have trouble maintaining the purity of their gene pool.
The Doctor still carries a telescope.
Lucie dons a spacesuit and goes for a walk on Phobos outside of the environment dome.
The construction robots are twenty-feet tall.
The Headhunter arrived on Phobos days before the Doctor and Lucie, but fell off a bicycle and knocked herself out. She doesn't wake up until they have left.
Location: Lunar Park, Phobos, 2589AD.
Future History: Lunar Park was originally designed to house a resort, including a hotel. Due to problems during construction, only the environment dome was ever completed. A group of hippies settled there, claiming squatters' rights. The Park is popular with extreme sports fans, known as "Drenies" (short for adrenaline junkies) who engage in orbit hopping, ice spelunking, bungee jumping, climbing and other pursuits.
The Bottom Line: "Maybe you're scarier than the monsters".