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.
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.
Hell Is That Noise: The Thief series has been well known for occasions with a rather disturbing ambiance.
Iron Woobie/Jerkass Woobie: Garrett, articularly after his chain of traumatic experiences in the final third of The Dark Project.
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.