Thief is an odd duck.
By 'odd', what I mean is, more or less, 'not very good, and in a very strange way'. I don't want to say that producing a new iteration in a well-loved series with heavy nostalgia value is easy, because there are always about twelve thousand fans waiting to scream that you've ruined it forever, but it does mean you have a frame of reference as to what said fans like, what they maybe think could be improved upon, etc. You have a lot of the groundwork with which to make an excellent game.
Worse still, what makes it bad isn't that it is overly difficult, or that its gameplay elements aren't implemented competently, or even that the plot is bad (although it is most certainly uninteresting). What makes it bad is that there's just not much there, in terms of gameplay, difficulty curve or, well, anything.
In every mission, you are dropped into The City, the game's steampunk Victorian/Late Middle Ages grimdark setting, in an area usually utterly indistinguishable from any other part of the The City. You are given a marker on your screen to get to: You are meant to do this stealthily.
Except, you don't really have to: That marker doesn't show your overall objective, it just shows the next point at which you won't be able to go backwards – and the next point at which guards won't follow you even if they know you're there. If those markers were far apart, that would still represent a considerable challenge, but each one is maybe twenty metres away from the last, at most. With such a short distance between them, you don't have to bother being stealthy: You don't have to bother with anything. You can just jog from one marker to another, casually alerting every guard in the area, because as soon as you hit that glowing mark, they'll forget about you and wander away.
It's a realisation I came to at the beginning of Chapter 2, and it kills the game utterly. A better stealth game might be able to weather that particular issue, but the stealth in Thief is, while not awful, often very flawed indeed. Frequently, if you're trying to take a stealthy path above your foes, the game will scream 'NOPE' and compel you to go onto the ground, so that you can dodge pools of light and throw pots to distract guards. It's not intentional, or at least it doesn't seem to be: The problem is, instead, that it is an incredibly linear game, and there's only one path you can take – and since the game desires that you be able to play it however way you want, that has to be a path that can be taken stealthily or can bring you into contact with guards to hit with your club.
('Play it however way you want' combined with 'wholly linear' and 'almost no difficulty', of course, seems to translate to 'abject game developer terror that anybody would protest that this game is too difficult, or that the difficulty curve is too steep.' I'm a big advocate of game developers providing a way for players to keep playing the story even if they're struggling with a particular part of the game – whether that's a system like Bioshock Infinite, where you return to life if you die but your slain enemies do not; or a system like Red Dead Redemption that lets you skip sections if you keep failing at them – but this is a very clumsy way of doing that.)
It's necessary, I think, for me to make a comparison with a much better stealth game. Dishonored, one of my all time favourite games, had a 'play how you want' idea behind it as well: But it backed up that idea with non-linear levels where you could take any of a dozen or more paths to reach your objective, and a range of powers which could vastly alter your play style. Even under the broad umbrella of 'stealth', you could take a high path along the rooftops and balconies of the city, circle around a building until you found a window into where your target was, and sneak in and kill them; or you could take a low path, maybe possessing a fish to swim through a plumbing system into a kitchen, and then sneaking your way upstairs, luring your target out, and killing them behind a set of screens. Or you could eschew stealth altogether, make as much noise as possible, and cheerily murder anyone in your path.
There was a genuine sense of urgency and terror when guards caught you too – battles were frenetic, confusing and alarming, leaving you either desperately trying to escape and hide, or desperately trying to summon a horde of plague rats to devour anyone who comes near you, depending on just how bloodthirsty you were.
The game provided you with an objective, obstacles, and self-contained but otherwise fairly open level maps, and told you to do what you liked with them. Youtube holds testament to some of the interesting things players did with that.
Thief, alas, never comes close to that, nor to the heights of its predecessors in the Thief series. Which is a shame: Done well, stealth games can be incredibly satisfying, and it grates that a franchise that was once so positively received should have such a poor entry in it.
Still, there's always Thief 5. Probably. Inevitably, really.