Have you played Thief? Or Splinter Cell? Or any stealth game ever? Seriously. Hiding in a dark shadow is entirely logical, realistic and believable. It's effectiveness varies based on distance. The further away a guard is, the less dark the shadow has to be for you to remain concealed. If the guard is close, the shadow has to be pitch dark for you to stay hidden. Deus Ex's implementation of light-based stealth was fine. The problem is that the AI was dumb and their general awareness was minimal. Again, this was an AI issue, not an issue with the stealth mechanic.
To reiterate, hiding behind cover is present in every stealth game. It is a fundamental aspect of stealth. It's not some new or revolutionary concept. It's stealth at its most basic level. Stripping away the light and sound mechanics doesn't make stealth more interesting. It makes it dumbed down.
Implementing light into an AI's visibility calculations opens all sorts of gameplay possibilities. You shoot out the lights, flip on the night-vision and voila, you can reenact Silence of the Lambs. Of course, the AI could use the same tactic against you depending on what augmentations or equipment they have. You could even use this against them. Shoot out the lights, forcing them to turn on night-vision, then shine a flashlight in their eyes, severely blinding them.
If the stealth in DX3 is stripped down to only cover, it is a step back.
I've played all of those games, and it's always the same. There has to be more to it than wearing black clothes and crouching in the dark. Sure, it's fun for a while, but no matter what game you're playing, it always works the same way : sit in the dark, wait for the guard, stun/shoot the guard, move on. How challenging! Hopefully, an interesting stealth cover system will be make sneaking around much more exciting.
I don't really see what all of the bitching is about. So what if it's different from Deus Ex? It may finally reinvent the stealth action game as something more than just creeping around in puddles of darkness. Maybe we can sneak around in a brightly lit building or during the middle of the day, for a change.
However, they are stepping off into such a different direction from the DX and DX:IW, I don't see why they don't just make a new IP. Guess it's easier to use someone else's ideas these days.