Thief was, in my opinion, one of the only games to ever do stealth right. Yes, Splinter Cell was very good, in that regard, not sayin anything else. However, Thief gave you exactly what you needed, and little to nothing else: a way to tell how visible you are (light gem), some kind of feedback on how loud you're being (one of the earliest examples of 3D audio with actual, nearly-believable propagation mechanics), and...the one thing a LOT of stealth-based, or stealth-inclusive games fuck up: your character isn't an unstoppable killing machine.
The latter point is a huge one, because if you can effectively tank all your enemies...what's the point in hiding? Why sneak around when you can just cleave a few dozen bastards in two with a sword, or blow them to kingdom come with an assault rifle? Thief both actively and passively punished you for fucking up the whole sneaking thing: guards would be harder to fool, bodies you'd have to hide, blood to clean up, guards hearing someone screaming as they die from an arrow to the eye or a sword to the spine, and on higher difficulties, outright failing you on the mission, thus forcing you to restart either from the beginning or your last save, should you kill. Hell, on the highest difficulty, the game outright forbids you from killing anyone outside monsters and steampunk robots.
However, given the tech we've got nowadays, I fail to see why we haven't had another stealth-based game that can rival Thief, both in gameplay and storytelling. For fuck's sake, we've got games like fucking Crysis, whose graphics can end up looking as close to photorealistic as we can get in a game...and, from what I've seen at max settings, it gets pretty fucking close. Why the hell can't we say, cut that graphics power requirement in half, have a game that looks good without having graphics that'd give any photographer a raging hardon for at least an hour, and use the resulting freed-up resources to make an epic stealth game?
Then again, we do have Thief 4 in the works, so maybe I'll see it realized, at long last. That is, if Eidos Montreal aren't too busy not showing us a god damned thing, not even fucking concept art. Its been some four fucking years, and we've gotten practically nothing outside a logo. I understand not showing things because they can be changed, especially at the early stages, but one: its been four god damned years, and two: give the fans fucking SOMETHING for waiting this damn long. A piece of concept art, sound clips, trailer, anything. Drag your feet too long and give us no real proof outside a few forum posts that you're actually working on this game, and most of the fans will likely end up walking away to other things, and you'll end up fucking your game right in the ass.