Thief also has two factions with which the player can align with or against - the Hammerites and Pagans, who come in handy from time to time. In fact, it pays to get friendly with the Hammerites in particular, at least in order to buy holy water from them to help deal with the undead.
The first two Thiefs combined sound and gameplay in a way that's never really been done before. As often as not, the player found himself guiding Garrett's actions by ear as much as by sight - to move when you heard a guard pass a certain point, or while they were distracting themselves with conversation. Deadly Shadows uses graphics, not gameplay, in ways that have never been done before. The lighting is real-time and dynamic, guards are often walking around with torches. The torches flicker, causing shadow coverage to move, and at higher difficulty levels, I could swear that the guards react not to Garrett's presence but at the sight of his shadow.
The AI is a step up over previous editions of Thief, but for the sake of gameplay, it still has some obvious holes. Kill the person somebody was talking with, they'll look around and chase after you, but quickly get bored or tired and simply give up, then act like nothing has happened. In fact, once you know the levels, the challenge is gone even on hard difficulty - just run towards your objective and run back to the exit and you're set. Similarly, Garrett, Master Homicidal Maniac has a much easier time of it than Garrett, Master Thief. If you kill everyone in the level, what challenge and believability the game has goes out the window.
Truth be told, Thief is only as good as the player permits it to be. If you play it as it's intended to be - stealthy, sneaky, paranoid, scared of anything that moves - it's a beautiful game. Lift up the outside coating however, and you can expose all the gamey tricks which make it a joke. Of course, it's those gamey tricks that permit players to escape punishment for their mistakes, but they can be abused to take the fun out of the game.
One of my own personal concerns was how ION Storm would deal with nonhuman entities. Previous games became increasingly reliant on the supernatural to give the player a challenge, to the point where it became an obvious crutch. ION Storm is more subtle about these foes in general. There are a couple of levels where they're Garrett's primary enemies, but for the most part they're a supplementary force and used with grace. You'll know what I'm talking about when you go to recover the Paw from the Pagans.