Demonic Spiders: Haunts. Extraordinarily creepy, cannot be black-jacked, best fighters in the game, extremely fast... even players who are used to fighting their way out of a bad situation will be extra-cautious around Haunts.
Spiders are annoying and the larger varieties (especially Chaos Spiders) can be pretty dangerous, but they aren't hard to kill. They are easily susceptible to a One-Hit Kill with a broadhead arrow, as long as they don't spot you (they have fairly rubbish vision, so it's really easy to kill them). A few slashes with the sword can kill them as well, but you're more vulnerable while doing so, since you have to stand close to them to deliver a blow.
Frogbeasts have an explosive capability, and have been regarded as a common nuisance.
Hilarious in Hindsight: Those who watched The LEGO Movie would find the game's Master Builder as a deity of Order of the Hammer whose strict rule over the city being opposed by anarchist-leaning Pagans in contrast to the movie's Master Builders being a name of a rebellious group in opposition to Lord Business' strict rule.
Iron Woobie/Jerkass Woobie: Garrett, particularly after his chain of traumatic experiences in the final third of The Dark Project.
Magnificent Bastard: Garrett, a former Keeper, is a master thief ostensibly out for himself who is continuously roped into world-shattering events against his will and defeats the enemies against him via sheer cleverness and audacity. Encountering the wicked Trickster Constantine, who seeks to consume the city, Garrett flawlessly orchestrates Constantine's destruction and escapes the vengeance of the Pagan faction. In the second game, Garrett learns of a plot by the evil Karas to annihilate all organic life and tricks Karas into destroying only himself. In the final game of the trilogy, Deadly Shadows, Garrett manages to destroy the power of a demonic beast from legend, the Hag and leaves her to the vengeance of the Keepers. While out for himself and occasionally ruthless, Garrett is always ready to show his enemies exactly why he is the best thief alive.
Memetic Badass: Garrett. He has joined the ranks of "Impossible thieves in fiction that can rob you blind no problem."
Fan missions, mods and other fan works :
Fan Wank: Since the storylines of many fan missions often extrapolate from established Thief canon, you're bound to bump into something based on WMG by the series' fans.
It's Short, So It Sucks!: The number one complaint about any fan-mission is "I wish it were longer!" Not to say that FMs are bad or short (many of them rival the original missions in length and complexity), but since most FMs are single-level, the fun can only last so long.
They Changed It, Now It Sucks!: Completely averted; almost any change to the status quo is seen as a brilliant innovation, especially when pulled off right.