The role of Vichy France is vastly downplayed with no sign of French military forces underneath the Nazis. The only sign they exist is an off-hand mention by Sean that the French military is fighting with the Nazis. This may be Artistic License - History to prevent killing French soldiers.
It goes beyond that: not only are there apparently no British, Dutch or Polish units stationed along the border, but there seems to be no French units stationed there either - in fact, there's no evidence of a French military even existing in the game beyond a few Foreign Legion prisoners you rescue in one mission.
The game manages to make quite a hash of WW 2, featuring weapons that were introduced into service years after it takes place. Most visibly are the MP44s commonly carried by SS troopers (introduced 1943), and the Panzerschreck (1943). France is also littered with V1 and V2 launch sites (the latter was first launched in 1944, in a bit of historical irony, it was aimed at liberated Paris).
There's also more minor ones in the form of the song "Feeling Good", which was written 20 years after WW 2, the cabaret song "Koop Island Blues", which was written in 2006, the 2004 Madeleine Peyroux cover of "Dance Me to the End of Love", originally written in 1984, and... let's just say most of the soundtrack isn't from the time period, they just sound like they would be.
Artistic License - History: Basically, the developers weren't trying to be historically accurate; they just went with whatever looked cool. Be sure to remember the mantra, because you'll be reciting it quite frequently if you have more than a passing familiarity with WW 2 history.
The in-game rendition of the Trocadéro Palace across the river from the Eiffel Tower was actually demolished in 1937 and replaced by the Palais de Chaillot long before the Nazis occupied Paris.
You are asked to obtain some antibiotics in an early mission. Whilst antibiotics existed before the war, they were not referred to as such until 1942, two years after the events of the game. Justified, for the same reasons the French characters don't speak French all the time.
There was also a bit of name confusion on part of the developers with regard to the German firearm designation scheme, the result of which is the fictional MP60, which is neither a Maschinenpistole (submachinegun, though it might as well be one in the hands of the Terror Squad), nor was it introduced in service in 1960 (and then presumably traveled back to WW 2 via a temporal vortex).
The plane that Skylar uses to insert Sean behind enemy lines is based heavily off the P-61 Black Widow, which did not see service until 1944. In fact, it wasn't even on paper during the game's timeline.
Bishop: "Have you ever heard of atomic fission before, Sean?" Sean: "Meh... I never was one for poofy cocktails."
Justified, since nuclear physics was still a fledgling concept by this point.
There's also the "What the fuck is 'radar'?" conversation, but justified here as well, since the British referred to it as RDF at the time, thus Sean would either know it by a different name or not know about it at all. Also, only the British made widespread use of radar at the time the game takes place in, so they would be keeping the details as secret as possible, for obvious reasons (Wilcox immediately responds to above along the lines of 'something very secret, you don't need to know'.
Border Patrol: Entering the war zones on the edge of the map, marked with stripes on the minimap, will get you attacked by planes which do much more damage than normal.
Boring, but Practical: Once you get silenced weapons, you'll likely be using them for most of the game, except when fighting the Terror Squad.
Wilcox: Glasgow. Probably the best voice acting in the game. His voice actor is from Glasgow
Bishop: King's English. Isn't great, but is at least believable.
Sean: Northern Irish. Sounds a bit fake, and occasionally disappears.
Skylar: King's English. Sounds ridiculous.
Broken Bridge: Subverted. There are Nazi checkpoints that won't let you pass unless you possess the right forged papers (= are far enough in the story), but you can just crash through guns blazing or bypass them via stealth.
Character Development: Veronique eventually evolves from a non-action bystander to a competent soldier and leader in her own right.
Cheese-Eating Surrender Monkeys: Completely averted. Although the player character is Irish, the French Resistance are your primary allies and are generally depicted as stone-cold badasses to the last man and woman. In the prologue, when the war has just broken out and Sean is fleeing the Nazi invasion, it's possible to see French farmers rushing out to fight the Germans despite being armed with nothing more than farm implements and old hunting shotguns.
Combat Pragmatist: Sean has no problem with sucker punching an opponent, throwing them off a tower or kicking them in the crotch to win a fight.
Cool Car: The Aurora. Many more count if you're a fan of 1940's style cars.
Special mention goes also to the Altair. It looks gorgeous◊, goes like stink, handles well and it's so easy to obtain that you can have it two minutes after the sandbox opens up. It will likely be your car of choice until you get the Aurora relatively late in the game, and possibly even afterwards due to its friendlier handling.
Cryptic Background Reference: Any conversation Sean has with Skylar & Vittore that does not refer to their current predicament, as well as any mention of Sean's past. Why can't Sean return to Ireland? Who are the enemies he met there? Why was he in Budapest when he met Skylar? What did Sean's father do that got him kicked out of Ireland?
It's reasonably obvious (given that it's explicitly pointed out in-setting that Sean knows how to handle explosives even before the first mission, and given how he goes ballistic when mistaken for a Brit) that either he, his father, or both were members of the IRA, and that he originally learned the skills he uses in the game intending to apply them against the British.
Deliberately Monochrome: Areas without the "Will to Fight" appear in monochrome with splashes of color here and there. Once Sean has sufficiently disrupted Nazi control over an area, that area stops being monochrome.
Double Entendre: In a flashback near the beginning of the game, Dierker tells Veronique that soon, the women of France will learn to appreciate the taste of a purebred German bratwurst.
The dialogue in "Deutschland Uber Phallus" has plenty of this.
Dressing as the Enemy: One of the approaches to completing missions, and probably the one you will end up using the most.
Easily Forgiven: If you murder enough innocent French civilians, the Resistance will turn on you and hunt you down... for all of ten minutes, after which they forgive and forget, no matter how many people you murdered.
Elite Mooks: The SS troopers (at least compared to the standard mooks) and the Terror Squad.
Everything Trying to Kill You: If you get the Alarm level up to 5, you're in deep trouble. The "alarm zone" covers the entire game map and there are only two still active hiding spots in the entire game note unless you've managed to get the perk that keeps hiding spots active even at level 5. Unfortunately, you need to escape a level 5 alarm (or spend 3000 contraband) to get it in the first place.; one at a nondescript farm on the northern-most tip of the map, the other in the bar at the top of the Eiffel Tower. While you're struggling to reach either, airplanes will appear and start strafing you wherever you go, in addition to the zeppelins, superheavy Wulf Tanks, and the Terror Squads shuttled around by APCs trying to kill you.
Fanservice: The game starts with a stripper wearing nothing but a bottom and pasties. Not only that, your main hideout is in the girls' changing rooms. And to make it even better, downloading the DLC will not only take away the pasties, you will also get 4 stripteases.
Femme Fatale: Inverted for Skylar, as she's on Sean's side. Franziska is a straight example, but she doesn't appear very often.
French Jerk: Subverted, as Jules, Veronique and Luc are all Sean's friends (though somewhat grudgingly in Luc's case). Jules lampshades it while he and Sean are sneaking into Doppelsieg:
"I'm French, I'm supposed to be rude."
Goomba Stomp: It's entirely possible to hurt if not outright kill people by jumping on them.
Hello Boys: The very first thing you see in the game is a stripper wearing pasties. If you have a new copy of the game, it came with a code to remove said pasties.
Historical Hero Upgrade: The real-life French Resistance was pretty Badass to begin with, but this game takes it to outright ridiculous levels - by the mid-to-late game it becomes a fairly regular occurrence for Resistance forces to meet the Germans in full-scale open battle and win, or at least hold their own competently.
Hyperspace Arsenal: Not quite as outrageous as in other games, but Sean can still carry far more weaponry and explosives than would ever fit into his bag or coat. Also, when he isn't wearing a coat with an inner pocket, he can pull an infinite number of cigarettes out of his ear.
I Cannot Self-Terminate: When the Nazis attack the Catacombs HQ and destroy a critical cavern wall, Luc is trapped under three huge boulders. After the Nazis retreat to get reinforcements, Sean and Veronique try to pry him free, but it's clear they won't be able to get him out on time. Luc urges Sean to shoot him, because he'd break under torture and would die anyhow. Sean can't bring himself to pull the trigger, but Veronique does it after giving Luc a Last Kiss.
Infinity+1 Sword: Any of the "Terror" weapons count as this, once you unlock them or scavenge them in the field. They all have enormous amounts of ammunition, can kill in only one or two hits and are generally very nasty.
Jiggle Physics: Skylar's rack jiggles so much, you might think it's sentient.
Veronique as well. At the beginning of the game watch as she runs to you and Luc. They sway with every step.
Ludicrous Gibs: If you drive your car through a herd of cows, they will explode into bloody chunks. Oddly, you will also get the same effect if you punch them.
MacGuffin: The artifact that Sean has to retrieve for Wilcox in one mission. It's mysterious, in a huge glowing chest, and Wilcox is very adamant that Sean must not learn what it actually is. Therefore, the player doesn't either, and after the mission it's never mentioned again. Though if one goes around the area you found it, there is some people chattering thinking it was Napoleon's treasure.
Male Gaze: The very first shot in the game is of some boobs. If you have the DLC, there is a toggle in the options to remove the pasties, leaving the dancers topless. Not that the pasties leave much to the imagination anyway...
Monumental Battle: The Nazis transform nearly every prominent Parisian landmark into a base of some form; and in order to inspire the areas nearby, Sean must rout the Nazis from them.
The Hotel De Ville is used as the Gestapo HQ.
The entirety of the Ile de la Cité (including Notre-Dame and the Conciergerie) is used as a prison.
The Panthéon has a massive howitzer mounted in its Dome and weapons caches stored inside.
The Eiffel Tower is used as a temporary command post by Dierker as the city plunges into revolt against the Nazis. Sean must "fight" Dierker on the third floor observation deck.
Mook Maker: Small barracks littered throughout France will spawn enemy troops if/when the alarm is sounded. Very annoying, especially since they're not considered freeplay targets and don't provide contraband (the game's currency) upon destruction.
In an interesting inversion, Resistance fighters can come to your aid out of buildings in areas where color has been restored.
Nazi Gold: You can find "Nazi gold bars" in certain crates.
Noodle Incident: Whatever it was that got Sean kicked out of Ireland, it involved a huge explosion and a lot of fire.
Oirish: Sean Devlin is sadly a perfect storm of Oirish stereotypes. His accent, allegedly that of a man from Belfast, is not even close to the mark. Unsurprisingly he's voiced by an English actor. Much of his speech involves faux slang such as "top o' the morning", "to be sure" and various other turns of phrase no man from Belfast (or Ireland for that matter) has ever uttered. And finally Devlin is of course an explosives expert with a love of violence, womanizing and excessive drinking.
Scenery Porn: Climb to the top of the Eiffel Tower and gaze upon all of Paris. Doubly so when you've inspired the entire city and are thus seeing it in full colour.
Screwed by the Network: The developer, Pandemic, was bought up Electronic Arts due to their success with Mercenaries, but the sequel to that game bombed badly. As a result, the dev team was broken up after being allowed to finish this title.
Another to Assassin's Creed, appropriately enough, in the form of a car called the "Altair", which is noted in its description as having the nickname "The Flying Eagle". There's also one for the vantage points - this game has scenic vantage points too, complete with the 'leap of faith' areas marked with birds. However, if you jump off of them, you plummet and pancake into the ground instead of landing safely in a conveniently-placed hay cart. It's even lampshaded by Sean on one occasion:
Sean's last name, Devlin, implies he could be in the same bloodline as Ewan Devlin from Mercenaries 2. (Considering both were made by Pandemic.)
Terror Squad soldiers look awfully like members of Kerberos Corps. While you can pin the trenchcoats, Stahlhelms, masks and machine guns on being part of SS, the characteristic shields worn on the left arm are a dead-on giveaway.
Sigil Spam: Swastikas, really. In reality, armbands were worn only by SA and SS, usually only with dress uniform. In the game, swastika is displayed on banners adorning most buildings and streets in occupied zones, armbands worn by all soldiers and markings on every German vehicle.
Splash of Color: In the Deliberately Monochrome zones, Nazis and some of their equipment will have red areas, typically their armbands. Resistance-affiliated people usually have some blue clothing, like Luc's blue turtleneck or Veronique's blue pendant.
Also applies to blood spatter and some car paintjobs (most strikingly for the Aurora).
Soft Water: Feel free to leap from the top of the Eiffel Tower: as long as you land in that 2 metre deep kidney shaped pond next to it, you'll be fine! Of course, it's sometimes funnier to deliberately miss it and splat into the pavement.
Sophisticated as Hell: Father Denis. He usually talks like you would expect a priest to do in this kind of setting, but he actually swears a lot when it comes to it.
"My son. In the name of all there is holy: Blow his fucking head off!"
"Go now, my son! The Lord will be my shield, but He expects you to save your own ass!"
Vehicle radios also play one of several songs when you get into them, including the original Nina Simone rendition of "Feeling Good". After a short while, they morph into background music.
Space Compression: And how. The game world includes pretty much all of France along with a small bit of Germany, with the northern and southern coasts included. About 80% of it is taken up by Paris, which is a five minute drive from the border with Germany.
Stuff Blowing Up: The trademark of a saboteur, of course, and there is a lot of stuff to blow up. Each major zone of Paris has hundreds of possible targets, and there's more out in the surrounding countryside. This is important for a couple of reasons: you get contraband (the game's currency) for every target you take out and missions become easier when the mission area no longer has those annoying sniper towers surrounding it.
The Alleged Car: In the beginning, the only cars the Resistance garages have. They are also the only cars with 4 civilians inside.
The Dev Team Thinks of Everything: Occasionally, the Resistance will contact you by having an NPC run up to you in the street and pass you a note. If you kill this NPC, Sean remarks:
"I guess that's what they mean by shooting the messenger. They can chat me up later."
When Sean smokes a cigarette, he usually takes it from a pack that he stores in the inner pocket of his jacket. When he isn't wearing the jacket, he has a different animation, taking the cigarette from behind his ear instead.
You get a "Mission Failed" screen RIGHT AFTER THE CREDITS if you kill Dierker during the Saarsbrüchen Grand Prix (Possible by getting the gold Mayhem Perk before sleeping in the Belle).
The Guards Must Be Crazy: The only reason for sounding an alarm is seeing and identifying an intruder, then blowing a whistle. Gunfire, explosions and heaps of dead bodies are A-okay with the guards, who deal with them by congregating at the site of the disturbance and performing a ritualistic group dance that consists of turning one's head from side to side while standing around in a half-crouched position for half a minute. Once the gods of watchfulness have been thus appeased, it's business as usual.
Sean can take the uniform of a sniper while being shot at on a sniper tower and the guards will stop shooting at him.
The Mole: The candidates are: Luc (arrogant bastard), Bryman (a bit too helpful), Santos (profiteer), Skylar (Femme Fatale), and The Kesslers (supposedly defected). The mole apparently has complete access to all the information of the French Resistance.
It turns out that Santos is the mole. This is revealed when the Nazis invade the Catacombs HQ near the end of the game.
The Obi-Wan: Vittore is very fatherly to Sean throughout the game, making it all the more painful when he finally dies.
Theme Tune Cameo: A remixed version of Nina Simone's rendition of "Feeling Good" shows up during the final mission at the Eiffel Tower, played by a broken Nazi general on a piano. It's quite probably one of the most poignant and haunting moments in the game.
A different remix (the same one used in the E3 trailer) plays during the credits after you kill Dierker.
Sean: "So... What's it like, being a Nazi and all?" Cpl. Spitz: "My name is Corporal Spitz... I have ein package for Herr Bauer." Sean: "Oh? What's in the package?" Cpl. Spitz: "Whiskey. Sausages... Porno Magazine."
Unusually Uninteresting Sight: Climb up to the tallest chimney with a group of Nazi soldiers nearby you can find, and then just stand there having a smoke. They don't even bother looking at you. Now walk a step or two so you jump down and grab the ledge where you were just standing, which causes the soldiers to start looking at you. Hilarity Ensues.
Video Game Cruelty Punishment: Killing enough civilians in a short timeframe will temporarily get you negative perks with more and more severe consequences, up to the point where Resistance members will hunt you down for 10 minutes.
Villainous Breakdown: Dierker falls apart when you confront him at the Eiffel Tower. He says that history will vindicate them for defending against the "mongrel hordes", tells Sean they're Not So Different, and ends, nearly crying, by saying:
Justified, as Sean and Luc are building a resistance from scratch and the arms dealers (if Santos is anything to go by) are a bunch of money-grubbing bastards.
Zero-Effort Boss: The last confrontation with Dierker is essentially you riding an elevator up the Eiffel Tower in order to execute him with a single shot, without him ever trying putting up any kind of a fight. He will even jump off the tower himself if you remain idle for some time, sparing you the trouble of killing him.