Breather Level: Especially in Thief Gold where you're coming from a number of rather long and tricky missions, "Undercover" is a simple and short level which not only gives you a perfectly accurate map for most of the level, but also starts you in a disguise that causes most enemies to ignore you unless you do something suspicious.
Fetish Retardant: Given the circumstances of the specific scene, Viktoria's brief nudity in Thief: The Dark Project is anything but sexy.
Fridge Horror: In "Return to the Cathedral," higher difficulties require you to retrieve a corpse from a dark, damp cellar. A little creepy on its own, but it gets a whole lot worse when you realize that corpse has been rotting down there over half a century.
Genius Bonus: Based on the names you see, the Hand Brotherhood of mages originates from Fantasy Counterpart Arabic Culture. The word "mage" itself originates from "magus", which was used to refer to Zoroastrian priests, a religion which also originates from the Arabic region.
Goddamn Bats: Zombies. They're one of the few humanoid enemies that can't be knocked out, and defeating them permanantly requires Holy Water and Water Arrows. On top of that, their grunts and groans are completely interchangeable, make it hard to tell when one of them is on to you by audio cues alone, and some of them like to just lie around disguised as normal corpses until they groan and spring to life when you come near.
The Craymen in the Lost City are replaced with an expedition of Hand Brotherhood mages in Thief Gold. However, the Thou Shalt Not Kill objective that is implemented in every mission with human NPCs on Expert difficulty was never implemented here, meaning this is the one mission in the game where Garrett can kill humans on Expert difficulty with impunity.
In the original release, it was possible to skip 95% of "Return to the Cathedral" by keeping the front doors from closing by sticking a skull in the doorway and then just leaving the way you came in after the doors get stuck trying to close on it. Thief Gold fixed this, however.
Narm: The infamous eye-ripping scene is utterly terrifying. Still, there are a couple of things that could take the impact away a bit, mostly involving the ways Constantine and Viktoria speak after they drop their charades. Not only is the Trickster a ham-and-a-half, but no matter how horrifying a monster he is, it's a little hard to take someone seriously when his aliases include "Honeymaker", "Jacksberry", and "Woodsie" Lord. The two of them refer to Garret as "Manfool", and have a rather odd syntax on top of it all, pluralizing words that honestly don't "needs" it.
The Hand Brotherhood was a pretty popular mini faction among the series' fandom, even though its elemental mages only appeared in one mission based on their faction. The only other mission they appeared in was a few minor cameos in "The Lost City".
Raoul is one of the fan favourites among one-off side characters.
The Woobie: In Thief Gold, Raoul is a nice fellow who lost his livelihood and accomodation due to corrupt schemers in the opera house's management, lady Valerius included. Given how well he's taking his predicament, he's quite the Iron Woobie. His mild insanity probably helps...