So far this game gets the Numbuk Official Seal of Ogre Approving Goodness™. It definitely is making my old Thief nerve tingle in recognition.
Unlike Thief, just being in shadows is not enough (especially at higher difficulties). Sticking to the shadows may help you move from cover to cover more easily, but you really do need to hide your silhouette as much as you can if you're going to be stationary a while. It's like a blend of Thief and the new Deus Ex game.
I'm going through the game on Hard difficulty, and on top of which I've challenged myself to "ghost" as much of the game as possible (leaving everyone alive/conscious but slipping in and out as if you were never there). If I do have to take someone out, it will be non-lethally. Which is pretty much how I played the original Thief games.
Yep, you can go through guns and swords a blazing or go in for some sweet stealth kills. But ghosting brings up a whole different set of challenges. Patience, timing, looking for good distractions to cause, as well as alternate routes. On top of that, it's much more intense because you can never truly breathe a sigh of relief knowing that guards are still alive and mobile. It's fun!
Plus the role-playing part of me knows it infuriates the bad guys far more to know you not only completed your objective and robbed them blind, but you did it without the guards on their payroll seeing a damned thing.
My only couple of gripes is that there doesn't appear to be a way to sheathe your sword and gun. And there doesn't appear to be a way to turn off floating objective arrows (I'll double check that one again later today).