Considered to be the game that started the stealth FPS genre. Worth playing and replaying. A great game, great story.
What makes Thief so good is the character, Garrett and his abilities. It pays off to sneak in the shadows and to slowly pad across the floor to knock out an unaware guard, or while hiding in the rafters above to create a diversion by shooting a fire arrow to knock out a security device or to drop a gas mine on a group of unwanted passersby, or to lockpick a chest as quickly as possible before being detected. Even though Garrett has a sword, it should only be used as a last resort for a standup fight with guards and creatures. The payoff is so much better when sneaking behind and backstabbing, or to shoot a broadhead arrow from afar - a quick kill, relatively noiseless and and sustaining no injuries (in the expert levels one has to limit killing and only cause a knockout with a blackjack or with gas).
The Thief world is set in a medieval environment with swords, arrows, castles and torches but is also splashed with sci-fi technology such as mines, flash bombs, rope arrows, security cameras and a city full of gears and electric lanterns. With each level, you begin to learn about the intricate balance between nature and technology and that certain factions are trying to cause the downfall of the other. As you progress you find Garrett in the middle of it all, whose only interest is in profit, but whose actions decide the fate of the city.
There are very memorable levels such as Down in the Bonehoard (where you have to rob a very large and trapped crypt among the undead) and The Mage Towers (you have to unlock the four elemental towers to regain an important talisman while avoiding / killing / knocking out its powerful denizens). What makes the game great to replay is that playing the higher difficulties changes the objectives and adds much more dimension to exploring all levels.
Thief Gold has some antiquated graphics (faces and bodies appear more polygonal) but the combat physics are pretty good, and the ability to use a variety of stealth devices to hide or sneak upon the unwary enemy makes it a great FPS. The story and the level design is done extremely well. If you haven't played it yet, then it is time to give it a go.