These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Hilarious in Hindsight: While playing an on-flight game in 'Fitton', Martin (played by Benedict Cumberbatch, from BBC's Sherlock, which hadn't been announced at the time) references 'The Hound of the Baskervilles'. Even better after series 2 of that show, as the context of the reference is removing one letter from books to make them different, and he changes 'Baskervilles' to 'Baskerville'... just as that show did for its adaptation of the story.
Stephanie Cole is going to be in an adaptation of "The Lady Vanishes," which was part of the Hitchcock game in "Qikiqtarjuaq."
In "Limerick", the pilots and Carolyn play a game of evil-sounding names. One of them was Russell Crowe ("Soon you will rue the day you dared to cross... Russell Crowe," said in an evil voice by Roger Allam, who is famed for playing over-the-top villains.) Later, Crowe was cast as Inspector Javert in Les Miserables (2012). Guess who originated the role on the West End stage? Yep, that's right. It's Allam himself.
In 'Johannesburg', the plane is forced to land before they've even made it out of Spain. Carolyn asks if they've "found a hitherto undiscovered warp-drive button." Benedict Cumberbatch, of course, went on to play the Big Bad in Star Trek Into Darkness.
Another one from this episode: Arthur compared Martin to one of the guys from Top Gear (much to his disgust, before realising that Arthur actually means Top Gun). Cumberbatch, eventually, appeared on Top Gear.
In "Vaduz" there's talk about a dragon between Martin and Theresa. Benedict later played Smaug the Magnificent, a dragon, in TheHobbit.
The episode "Ottery St Mary" (particularly Martin's refusal to fly with a hypothetical otter in the flight deck) becomes even more hilarious if you're familiar with a certain meme involving Benedict Cumberbatch and otters.
This is not terribly surprising, given that Ottery St Mary (and the otter obsession it started in parts of the fandom) was the origin of this meme
In Name Only: The film Quest For Camelot mentioned in "Cremona" doesn't seem to have anything to do with Quest for Camelot (possibly because it wasn't released under that title in the UK). For one thing, it's live-action, but there's also the detail that "Excalibur" is apparently a Living MacGuffin (and turns out to be a vampire).
Launcher of a Thousand Ships: Martin appears to be the fandom bicycle judging by the fanfiction (not helped by the fact that he is played by the very attractive Benedict Cumberbatch). The most popular pairing starts with Martin/Douglas, then Martin/Arthur and goes from there. There are a couple of Martin/Diego (from 'Johannesburg') fics, and Martin/Molly Hooper and Martin/Henry Knight are both becoming popular.
Periphery Demographic: Cabin Pressure is a fairly gentle and traditional Radio 4 comedy, Radio 4's audience being memetically middle-aged, middle-class and terribly English. Presumably thanks to the presence of Benedict Cumberbatch in the cast (as well as the program being good enough to sustain a devoted following, of course), the show has attracted quite a fandom on the notably international, all-comers, young people jamboree that is Tumblr.
The Woobie: Martin, mostly due to his luck or lack there of.
Most obvious in 'Qikiqtarjuaq', 'Fitton' and 'Limerick' — aside from the fact that Martin had to retake his CPL seven times, with even his own family not believing he'd manage it, he also has to live in the tiny attic of a student house, and because Carolyn doesn't pay him for being a pilot, his only monetary income is the jobs he takes as a stupidly cheap man-with-a-van which he has to fit around his MJN flights.
A passenger makes him cry in 'Boston.'
Arguably Arthur in 'St Petersburg'. His frantic attitude and desperate desire to please his father are completely out of character, and even Martin and Douglas notice it.
Martin: What’s he like then, Mr Shappey?
Douglas: I don’t know, I’ve never met him either, what’s he like Arthur?