YMMV / Doctor Who S40E1 "Space Babies"

• Crosses the Line Twice: The idea that your entire identity could be overwritten through one careless action after going back in time is daunting to say the least. Fortunately, not only is this a reversible process and a brief throwaway Brick Joke, but Rubathon Blue looks and sounds utterly ridiculous.
• Epileptic Trees: The way that The Doctor mentions about "visiting" Star Trek sounds an awful lot like another crossover between the franchises could happen. Or could also be interpreted as some day visiting the set of one of the Star Trek series during filming. A crossover in a slightly more 'realistic' sense, perhaps.
• Fanfic Fuel: The Doctor heavily implying that the events of Star Trek are real apparently came of RTD’s love of the other franchise and desire for Doctor Who to cross over with one of the Star Trek shows. He’s laying the groundwork of the other show’s canonicity here in case the crossover opportunity presented itself. Until then, it’s opened the door to crossover fanfics.
• Fridge Brilliance: All those unaccounted for times the Doctor and companions might have thrown history out of kilter through triggering a Butterfly Effect have been smoothed over by a setting on the TARDIS made to specifically block that. It stands to reason other TARDISes have that feature and the occupants using them could avoid this problem as well. The fact that this switch was turned off for the Doctor's TARDIS—which presumably happened either when the control room changed or when Donna accidentally spilled coffee on the console—would also explain why the Doctor and Donna were able to accidentally change the word "gravity" to "mavity".
• Fridge Horror: Sure, the Space Babies are cute, but think about their situation: they have the emotional intelligence of six-year-old children, but the bodies of babies. They can talk, but they can't walk, control their toilet functions or even blow their own noses. They have had zero contact with adults since birth (something critically important for healthy early years development) and have spent six years in terror of the monster living in their basement. Let's hope Jocelyn and the new colony can help undo the damage, or these kids are going to be messed up as they get older.
• Improved Second Attempt: The episode's twist that the monster stalking the space station is actually made out of discarded human waste—in this case, snot—may remind viewers of the infamously panned twist from "Sleep No More", where the Sandmen turned out to be made from the eye boogers of those exposed to the Morpheus signal. However, unlike that episode—which played the monsters and its premise completely seriously and thus made the twist come off as pure Narm—this episode has a much more tongue-in-cheek take on the concept and a more comedic energy overall, helping the reveal land better (especially because of the Doctor and Ruby's over-the-top reactions to realizing what they've been surrounded by this whole time).
• Older Than They Think: The Star Trek joke (see above) is funny, but there actually has been an officially-licensed crossover between Doctor Who and Star Trek. Assimilation was a crossover comic produced by IDW in which the Eleventh Doctor and the Ponds land aboard the USS Enterprise at the time of Star Trek: The Next Generation, and become embroiled in a conflict with the Cybermen and the Borg, who have joined forces based on their shared goal of assimilating all life.
• Special Effect Failure: Due to the decision to use real babies and have them speak with CGI mouths (see below), the babies look very strange. The top halves of their faces sport the wide-eyed expressions of curious infants, while their mouths speak and emote like six-year-old children. The effect is unnerving.
• Squick: The Boogeyman is literally made of bogeys, AKA snot.
• Unintentional Uncanny Valley: The CGI mouths used for when the titular Space Babies talk feel really off in certain shots. Special mention goes to Eric's final scene with Ruby hugging him. He tells her he loves her, but the baby "actor" is clearly not comfortable being held by this strange woman.