One of the rare inversions of the trope. The actual London Underground is substantially larger than it is portrayed in the game.
Action Girl: Player characters can be female. Back Story examples include Lyra Darius and much later, Jessica Summerisle, who was only a child when the invasion began but later grew up to be a ranking Templar commander, like her grandfather.
Affably Evil: Murmur. Even after he reveals he is a demon, he still displays the same polite personality he has exhibited through the entire game.
And Your Reward Is Clothes: Played around with, as you can get armor, weapons, weapon mods, or consumables as rewards, in addition to money and reputation. Even weapons get pimped out and display their equipped mods.
Subscribers were rewarded with dye kits, which changed the character's clothing color scheme to various presets. Some of the rarer ones also had highly sought after stat-boosts.
Artistic License - Geography: Due to randomized level architecture, many areas you visit don't look at all like their real-life counterparts, with exception of fixed areas like Piccadilly Circus.
"Blind Idiot" Translation: The online version released by Hanbisoft falls into this. Even quests that were from the original game, which was in English, fall under this because for some reason they decided to replace the quest text with the same quests, but minus any references to the overall storyline. For example, you still get the quests to clear tunnels of enemies, but no one mentions why you were going through those tunnels in the first place.
Cast from Hit Points: The Summoner's Blood Link ability which heals the main Minion. Also, both Cabalist classes have a skill which can exchange Life Points for Power Points.
Cat Scare / Nothing Is Scarier: Slain monsters are removed from the map when out of players' range, then are re-spawned and re-killed when returning. The net result is that death cries echo most clearly, and sometimes startlingly, when the map is empty.
Dark Is Not Evil: The Cabalists use the powers of the demons against them, though they claim to be on the side of good and have often proven themselves as such. Evokers use their offensive magics, and Summoners create elementals and enslave demon minions. Many regular humans distrust them, especially theTemplars.
Demoted to Extra: Murmur, who was originally the first character you met and remained relevant through the entire story, basically disappeared completely in the retranslation. He went from literally being the most developed character in the entire game to uncredited appearance in a couple cutscenes.
Devil in Disguise: The story's narrator / hero's sidekick Murmur turns out to be the Duke of Hell with the same name. To be fair, the name's a lot more subtle that Lou Cypher, and isn't likely to mean anything to players without a background in demonology mythology/Mega Ten fan.
Dual Wielding: The Blademaster class has this as one of its gimmicks. Evokers have Dual Focus, which allows for wielding of two more powerful Focus Items.
Dysfunction Junction: Almost everyone who isn't suicidally incompetent is a raving psychopath. You'd think that in a demon-possessed future the latter would be a requirement for survival, but there are surprisingly many of the former still living. And sending you on Fetch Quests.
Enemy Summoner: Zombie Lords summon zombies, and can buff any zombie types, including each other. Flesh Golems litter the ground with Death Maggots, and can spawn those and Zombies on death, while Wurm creates hordes of Death Maggots and fortunately takes them with him when he dies. Soul Reapers and Orbiles summon Polyps, and Bedlam can summon them all endlessly. Dunder Liches, including Flaw, summon 2-3 Fire Elementals.
Face Full of Alien Wing-Wong: Technician 314 gets attached to the Oracle. It doesn't stop him from talking, and when he's not speaking in tongues, he complains with every breath about the ancient demon with a deathgrip on the front of his head.
Fighter, Mage, Thief - Templars are characterized by raw combat prowess, Cabalists cast direct damage and debuff magic, and Hunters' skills include evasion and critical strikes.
First-Person Shooter: Default view is a third-person chase cam, though when using exclusively guns you can zoom in to a first-person view with a smart reticule that shows the current accuracy of your guns based on movement and firing speed.
Gatling Good: Of a sort, there is a particular gun with three barrels, each drawing on its own magazine.
Giant Mook: Variously-sized Giant forms of the usual Mooks appear, not much more difficult than the standard. Champions and other Rare versions get the larger size with a notable power upgrade. A few bosses (Bedlam) are King Mook.
Inverted by mini Mooks, who actually are more difficult due to being smaller, speedier targets. In maps combined with Goddamn Bats, they can take you down from the kneecaps while you're targeting the more conspicuous attackers swooping down on you. AoE attacks are your friend.
Guns Akimbo: Pistols in each hand for Hunters (Marksmen and Engineers) and Cabalists (Summoners and Evokers). Pistol and Focus Device (magic-enhancing gloves) for Summoners, while the spell-spamming Evokers can equip two Focus Devices with the appropriate skill. Templar classes can equip a grappling pistol and another pistol, but the effectiveness is questionable.
Hellfire: Omnipresent, wether it be leaking through cracks in the pavement or being projected by Cabalists.
Magitek: Hunter classes can call in an artillery strike using a certain skill (even while underground or in Hell), which is a cause for Fridge Logic until you realize how it works - the shell is fired and then a Cabalist teleports it to the target location.
Cabalist "Focus Items" are expressly this, being demon magic-imbued gauntlets that amplify and project the powers of their users, essentially, spell-firing guns. Evokers can dual-wield them and depend on them for success, as their spell power is directly proportional to the damage output of their Focus Items.
The Musketeer: It can pay to have a ranged weapon on hand if you are a melee character, and vice-versa.
Night of the Living Mooks: Zombies are everywhere. They are weak, but very numerous and can be buffed by Zombie Lords. Other, stronger types of undead are sometimes encountered.
No-One Could Have Survived That: After making an obvious Heroic Sacrifice to save a young Jessica Summerisle in the intro, Lyra Darius turns up alive, well and not having aged a day since, in Liverpool Street Station. She doesn't understand it either, due to Laser-Guided Amnesia. She offers a quest chain that concerns the truth of the matter.
Obvious Beta: One of the biggest complaints about the game and a prime cause for its ultimate failure. The fact that the patches quite rapidly solved most of these problems tended to be glossed over by critics.
They probably couldn't gloss over the crash-prone resource hog that remained after patch.
The Plan: Hunter Jon has a date planned with another Hunter he has a crush on, knowing full well she intends to use him as monster bait to collect a few kills during the outing. He sets up elaborate plans to establish the appearance of success for her without actually putting himself in danger.
Rag Tag Bunch Of Misfits: The survivors of the whole, but also specifically the Hunters, who are made up of former soldiers, secret agents, Private Military Contractors or what have you. They waged a Secret War on behalf of their governments for many years- until the demons came. The Hunters were quick to share knowledge and technology with one another and dedicate their skills towards the cause of humanity's survival.
Ruins of the Modern Age: The streets of London are a mess, most buildings are collapsed or at least burnt out, wreckage and debris lie everywhere, and Hellfire seeps through cracks in the pavement.
Standard Status Effects: Stun (no movement, attack or skills), Ignite (lose 5% of your Hit Points per second), Poison (prevents healing while in effect), Phase (halved defense and attack), Shock (no skills for the duration), which may happen if hit with the appropriate damage type. Some of the Engineer's support Bots increase allies' speed and slow down approaching enemies.
Underground Monkey: Mostly averted, see Character Levels above. Some models are reused however, but a recoloured monster is a bit more than just a beefier version of the weak "prototype" - Poison Zombies, for example, leave a trail of poison damage field; Black Skeletons leave a fire field when killed.
Unexpected Gameplay Change: Several. The already mentioned tactical mission. The Big Gundown level - you have to man a gun and shoot down a huge... thing otherwise outside of range - normally. While you are still damaged and forced out by Polyps. The Angel passage level, described above.
Unwinnable: Among other such Game Breaking Bugs, it was possible to leave an area called "The Exodus" before having completed it. It was then not possible to get back in and thus, progress through the main storyline. Due to the way the autosave logic worked, this probably meant you had to restart the entire game after already having progressed quite a ways.
Weaponized Car: As shown in the intro, survivors sometimes travel the surface in armored combat vehicles. They show up in-game but aren't ever usable.
Womb Level: 314's mind.. Also, the Exospector level.