Great third addition to the Thief series. Instead of simply going on to the next mission after completing one, you go back to the city. Here you can do everything from purchasing arrows and equipment, to stealing from the citizens, to picking on the guards.
Plus, the story is a bit more open. Not sure if this is a spoiler or not, but at one point you can choose whether to become friendly toward the Hammerites, or the Pagans, which affects the rest of the game after that.
Instead of a sword, you have a dagger. Makes backstabbing much easier.
No rope arrows, but you can eventually purchase climbing gloves. These are AWESOME, if you can figure out how to use them. :D
Oil vials. Can be ignited to make a temporary, impassible wall of flame, or can be used to make guards slip, slide, and fall down elevator shafts, if you know what you're doing. Awesome.
The ability to go 3rd Person in the game is interesting -- at first it detracted from what I was used to in the Thief series, but I quickly got over it.
You also now have the ability to press up against a wall, suck in your gut, and thus become almost invisible in a dark hallway. Reeeeaaaally creepy putting this to the test for the first couple times.
Lockpicking is a little more immersive and realistic, as well.
Needs some tweaking to make it work on newer machines (opening cutscene is an instant crash for me). One thing that sucks is loading screens within missions. But all of these issues are easily fixed. Just look up some mods for Thief: Deadly Shadows and you'll be set for an amazing time! :)
9/10. Would be 10/10 if there were rope arrows and no need for mods to patch problems. Still, WAY better than the new "THIEF" knockoff (aka Dishonored 2).