Some great additions for the series, and some drawbacks too
First of all, this is the third in the planned Thief trilogy, and the fact that it's here is good. Stephen Russell is back to voice Garrett(as well as guards etc) along with other instantly recognizable(and excellent) voices, Thief style FMV's are back and they're as atmospheric and other-worldly as always, although mixed in with some that appear to be in an in-engine style, there's still the haunting ambient sounds to keep the tension up. All these things are familiar to seasoned Taffers and help bring the unique Thief atmosphere back, and should draw newcomers in to the game.
The graphics are excellent, although technically an improvement I wouldn't say that they're necessarily so, as the originals looked sharp and the graphics did the job very very well(The Dark Project even ran smoothly on my ATI Rage 2MB onboard graphics!). Deadly shadows still looks excellent(there's much less right angles too) and this is failing to mention the real-time shadows; which are a great addition to the game, and directly effect gameplay, you can now hide in the shadow of a door left ajar, or a pile of stacked boxes etc.
There is now a much more open-ended element to the game, and even some side quests that effect your standing with the factions. The City itself is very much part of the game, patrolled by the City Watch and wandered by valuable adorned passers by. There's fences and blackmarket shops throughout the City of several types - some will buy some types of goods from you and others will buy others. In turn some shops will have equipment or items in stock that other's won't. You'll have to travel across the City to get to the areas of the next mission, and this is where the open-endedness really comes into play, stop to pickpocket a passer-by, break into an armoury or a closed up tavern, or pretty much visit any area you want.
The missions themselves are varied, from a traditional manor/castle type estate to a zombie infested ghost ship, pagan settlements, a spooky maze like asylum etc, one of my favourites was the sunken city inhabited by the lizard-like Kurshock. The later levels are more difficult, and achieve a very good level of tension and scariness, in keeping with the original games. The plot quickly becomes a driving force behind the missions and, as always, the Keepers are very much involved.
The AI will notice extinguished torches and investigate missing valuables, comrades and open doors. A small point but it's now longer possible to stand in the shadows and lean into a well lit corridor unseen, more realistic and a good point in my book.
My main gripe is the movement, which is somewhat clunky and slower to react, once you've pressed a key you've got to wait for the model to do it's thing. For example, say you're having a look around and decide to set off in another direction, the model will turn to first to face the same direction, and move slightly to one side while doing so - this resulted in falling off a close ledge a couple of times early on. Leaning also feels rather haphazard. I did get stuck in the scenery a couple of times too. That lot all said, I got used to it pretty quickly and was having too much fun to notice.
Load zones, most missions are broken up into two areas and the City is broken up into several areas, now this didn't bother me too much, but one drawback is that it's easier to run through one and leave a band of cursing guards on the other side, also they can sometimes still be waiting for you when you return, this detracted a bit from the experience.
The design of the game seemed a bit easy in places(not the AI). No swimmable water, entering deep water results in instant death. Rope arrows, while highly improbable, were fun to use but have been replaced with climbing gloves. The HUD is more obtrusive and has some unnecessary curly iron work. Now there's a lot of tweaks available on the web that address a lot of issues, but personally I thought I'd just play the game as is(and moan about it afterwards).
The missions are much smaller than the originals, but a lot of effort has gone in to making the levels not feel small.
A small note but the guards will now get tired and pause while chasing you, this makes it easier to get away but may be down to the smaller levels.
The Bottom Line
There is a fairly long list of improvements and drawbacks, perhaps the drawbacks are a little longer on my list but overall the game is true in many ways to the Thief series and is a lot of fun to play through.