Interface
Not the same old interface
One thing Thief definitely had going for it was the interface and play control. Once you master the short tutorial that comes with the game, you're pretty much prepared to go out and start your quests. Since you're a thief, the shadows are your friends. There's a little meter in the bottom center of the screen that measures how visible you are. If you're standing in a darkened area, the meter becomes black. If you start to move it lightens just a little bit, until it gets fully brightened when you move out into the light. Holding out a weapon at the ready makes you more visible. Crouching will also reduce your visibility, and has the added bonus of making your footsteps quieter.

This lighted area flickers
Keep your ears open
Speaking of footsteps, audio cues can betray your presence to a guard or monster. If you're careless about noise, they'll find you just the same as if you walked out into the light. With that in mind aspiring thieves learn quickly to note what kind of ground they're stepping on. Metal gratings make the most noise, followed by wooden platforms and concrete. Plush carpet or grass is the easiest material to skulk around on. If you intend to sneak up behind somebody for a backstab or a pickpocket, you better pay attention to the terrain you'll be treading on to get there.

Run across when the light flicks off
Peekaboo
One of my favorite interface features was the "look quickly" button. In my interface I bound 'Q' to look left and 'E' to look right. These look quickly buttons allow you to peek around a corner without being spotted. Simply hold down the button to keep peering around and release the button to hide back behind the corner. Such a feature would probably not have a place in a fast paced game of Quake but it really makes sense in the slow, cerebral style that characterizes Thief. Just don't hold the button down too long and stare at your targets. They may notice you spying them if you're in a lit spot.