With this demo you have succeeded in all three of the most important ways:
Great concept - a game mechanic that, while not without precedent, has never been this well executed in an FPS, to my knowledge.
Great mechanics - the physics manipulation makes for good puzzles, the controls work and it actually makes sense!
Great game world - it's got a dystopian, 1984 meets Portal thing going on,, which works well. You quickly learn all you need to know about the world and the character's place in it, but are also given the opportunity to learn more through information you are left to find yourself, which I appreciate.
Of course the graphics, level design and enemy behavior could be elaborated on a bit, but as a lean, efficient proof of concept this is flawless.
I'm tipping you 20 bucks because I want to see a full game!
Edit: Well, I WANT to tip you 20 bucks but there's no means available on the game page. Show me a secure method of payment and I gladly will.