I like Deus Ex because it's relatively forgiving. Most of the time, the guards will go "Huh? What's that?" Before blasting your guts full of lead, and since your guts are quite soft in that game (especially at the highest difficulty, which I still found fun), this is a good thing. Also, it allows for players who don't have lightning-fast reflexes.
Oh, and none of that "You were detected... GAME OVER!" crap, like in certain other stealth games. Plus you don't have to kill (most) enemies, you have some freedom to solve many of the game's challenges in different ways, and the story and dialogue are pretty deep, though it borrows a lot from here and there, granted.