One word for you: atmosphere!
First of all, the story is very believeable and it is told in a great interactive way. Cutscenes are merely to support the story, rather than to tell it. You learn the story of thief throughout the game by reading books and notes, overhearing people talk by eavesdropping beneath a window and stuff like that - but be sure to remain in the shadows at all time. You litterly become part of the story. But apart from the main story there are a lot of ambient-like tales. You can read things like fairy tales, hear people pray to their Gods as they recite their sacred hymns, follow the soap of a guard with a bad laceration or keep track of gossip. Everything that is mentioned in the story is related to the big picture and makes you feel you are actually there.
Ofcourse the game follows a scripted story, but it doesn't really feel like that. There is plenty of room for the player to pick up on side missions,go out and rob some houses or mug some people on the streets.
The sound is one of the best I have ever heard and makes the whole experience deeply immersive. Truly, I've never heard such well chosen audio effects in a game. This is not to underestimate; I really think this is the best aspect of the game. The voice acting is very convincing, you can hear the right type of emotions in their voices when they talk, reagrding the situation they're in, but what really sets the tone is the extraordinary ambient sound fx. We're not talking just footsteps here, but a wide array of really well recorded and mastered (ambient) audio. The sound's ultimately what drags you right into the middle of the whole experience. It is not there just to fill up some silent gaps, but actually makes sense as it contributes in a very high level to your suroundings. The music is woven into the game at the same professional level as the fx. It never is very present, but you would miss it if it wasn't there. It is highly subtle playing in the background and swells to a climax at the right times.
The graphics also are superb. Sure, it is an old game but there is a difference between good graphics and beautiful graphics. Thief has got good graphics, which means they are not there just to be pretty and entertain your visuals but they really do sketch out this surreal medieval world which gives you an impressive dark feeling. This is great! As you being a thief and all. The whole game is played in the night, but this is never annoying. For the sake of the engine, the game is best to be played in the dark - if you do that, you immediately get sucked up into them. I also suggest a headphone and dimmed or no lights.
The gameplay is good, but I have to be honest here. It feels somewhat clumsy from time to time. It is possible for you to get stuck behind objects or in narrow crevices. Most of the times you can wiggle yourself out, but because you have to be as stealthy as possible, this can be somewhat annoying. Another not so good aspect of the controls shows up in climbing and walking in tight high-up places. There have been a couple times I was pulled down by gravity due to clunky controls and finding myself in the midst of two enemies. It even boils down to a point where you have to shake Garretts head in order to pick up that one piece of loot. Sometimes the auto object selection turns out to be an enemy as well.
Overall, the controls are not terrible and the game is really playable, but in this field there is definitely room for some improvement.
The AI turned out to be not completely stupid and that was a nice surprise for a game released in 2004. Actually, they are pretty good. Sure, you will see them run into a wall once in a while but most of the times they'll do a nice job. They miss other patrolling guards, they do spot marks of blood and they will investigate. They even become suspicious when they miss certain items you just nicked behind their back. You can fool them by tossing all kind of stuff around like throwing a pewter tankard in the corner of a room to make your opponent go take a look. There's actually quite a lot of stuff you can pick up besides your daily loot.
If you've been making to much noise or when you expose yourself too much, you're screwed. They'll hunt you down and can do some good damage. A one on one fight can turn out to be in your favor, but make sure nobody sees or hears you fighting as they will come over to check out what's going on. Fighting against multiple opponents is never a good idea in this game so you better beat a hasty retreat.
If you get caught in the act by a civilian, they'll run out to the nearest guard and warn the officer. Also make sure to stash your bodies in a dark place where no one goes for it will trigger them to figure out what has happened and somehow they always think it was you who did the crime - even if you didn't do it. This is where the AI is a little bit off.
If you add all of this together you come up with one word: atmosphere. The tention oozes of your screen and there are moments your heartbeat will most certainly rise. You can feel the breathing down your neck when enemies stalk you. The game offers some awesome missions which may be not advisable for the faint hearted, you will get thrilled.
Despite some of the downsides I mentioned, Thief III is the most immersive game I have ever played. Everything adds up to a great experience: the sound, the graphics and the story are so incredibly well mixed it really gives you the feeling of being Garrett rather than you are playing a game. Over the passed decades I must have played hundreds, maybe even thousands of games. Many of them impressed me and gave me great pleasure but Thief III is the first game that really made me feel I was part of its world, no... I was in its world. You become the thief, you are the thief.