Thief is the most immersive game I've ever played. It gave me my highest experience of total encapsulation within a game. I got so immersed in this game I think I even started staying in the shadows more as I walked around in real life. In addition, my memories of what happened feel very close to being my own memories as opposed to memories of playing a game.
I remember picking a lock on a door of a hallway, and a guard suddenly opening the door and coming through. I spun around behind the opening door and whipped out my blackjack, neatly rapping him the back of head as he walked past. The whole thing took less than a second.
In 1999, you just hadn't experienced anything like that before. And no other game since as achieved the same level of stealth quality. I think the problem with newer stealth games is that they cater too much to the FPS crowd and allow your character to be too strong at direct combat. In the first Thief game, you could fight one guard with a sword and come away with a medium level of damage. If you faced more than one guard, or the guard called for help, you had to leg it. There was no way you could stay and fight. In Thief III, you could fight through the game like it was Dark Messiah if you wanted?.just not the same experience. If there is no real threat from discovery, the thrill of the sneaker is lost.