A genre-defining brand of stealth, which it borrowed from its predecessors, and super atmosphere, make Thief 3 a must!
First off, I'll tell you about the setting. Thief 3 takes place in the same dark fantasy world as the first two games. It's a pretty original setting as it combines steampunk and medieval elements into one unique world. The setting works great, nothing feels forced or out of place. Power is concentrated in the hands of the Keepers, a secret society of which Garret (the protagonist of the series) was a part of, but not anymore. Other factions include The Hammerites (a theocratic religious group), The Pagans (a group of outcasts living underground) and a few minor factions like the pirates, the undead and of course, the City Watch.
In terms of game mechanics things haven't changed significantly since The Metal Age, but some elements have been lost along the way. For example, you no longer get to use rope arrows, which is pretty disappointing, considering the fact that you play the role of a thief and you might need to reach some places otherwise inaccessible. You do get some special gloves that allow you to climb certain surfaces, but quite frankly they feel kind of clunky. Rope arrows would have been better. The above could be considered nitpicking, so to talk about a real problem I'll mention the simplified level layout and generally smaller levels when compared to previous Thief games. That has a direct impact on mission complexity, which definitely isn't what it used to be, and this is probably the biggest shortcoming Thief 3 has.
Another issue is the AI, which I feel could have used a few enhancements, as enemies are quite easy to evade even on the highest difficulty level. NPCs are quicker to dismiss the little sounds you might make (some times by accident), and are not that eager to investigate louder noises either. They just seem more lazy than before, which is shame, as this impacts the gameplay in a negative way, making Thief 3 feel easier than its predecessors.
The bulk of Garrett's equipment has been ported successfully into this third installment: you have your faithful water arrows for putting out torches, broadhead and fire arrows for damaging and killing enemies, the noisemaker arrow for creating diversions, gas arrows to put enemies to sleep etc. A few other things like flash and gas bombs, holy water or health potions are also part of the equipment locker. Also, your faithful blackjack makes a comeback, but Garret has dropped the old sword for a small dagger, which feels like a logical move. Finally, the light gem also makes a comeback, having the same function as before, though now it's also accompanied by a compass.
Completely new is the ability to explore the city between missions. Now you'll be able to visit shops and sell the loot you have stolen, buy equipment, or simply enter random houses and steal stuff. Quite frankly, this is a great idea and the implementation is good enough, though environments feel kind of small. A problem that I encountered is related to the looting aspect: it's rather easy to make a lot of money and sell prices for loot is rather high, so you're always going to have excess money, and thus excess equipment. It would have been nice to be able to use that excess cash to buy some special upgrades for Garret, the ones that are available are pretty limited in number. But overall, in terms of game mechanics, the developers took the conservative route, and that was wise of them.
Thief 3's story focuses on a prophecy that speaks of dark time to come. It basically boils down to doing various missions that will help you decipher the mystery surrounding it. It's not the best of stories but it gets really interesting towards the end. The problem is that it picks up slowly. Overall, Ion Storm could have done better (considering the great job they did with Deus Ex's story) but it's still one of the good aspects of the game.
In terms of graphics, the development team has done a great job. Thief 3 uses a highly modified version of the Unreal 2 Engine, and its qualities show. The game runs smoothly while also looking exceptional. It does dynamic shadows and light very well while also rendering textures at good quality and displaying a variety of SFX like bloom and glows. The graphics are good even by today's standards, and serve as an important immersion factor. It's too bad that environments are a bit small. Still, it's not going to detract from the fun of stealing people blind.
Thief 3 is also top quality when it comes to sound. Ambient sounds are pretty good, things like doors, mechanical devices or footsteps sound great, while voice acting is generally good. But the music takes center-stage here. The soundtrack is composed in the good old fashion of Brian Eno's ambient pieces, with a bit of an ominous feel here and there. I personally love this sort of music, and I think anyone can see that it suits the game perfectly.
Conclusions? Thief 3 might be the weakest of the series, but it's definitely a worthy addition. It has a few odd choices here and there, and it's also an easy ride, but the atmosphere, generally good game mechanics, and the setting more than make up for it's shortcomings. Thief fan or not, you must try it!