I realize there are many such threads in the Penumbra Tech Demo forums, but I thought it might be more appropriate to have a suggestion thread here that pertains specifically to the finished product.
That said I thought I'd just toss up a few ideas just so I'll know they've been mentioned.
1. Lean modifier key. If you're trying to create immersion through mouse driven actions it makes sense to me that leaning would be controlled with the mouse as well. As it is in the tech demo it's very slow and robotic and not very immersive at all. I think it would be neat if there were simply a lean modifier key whereby holding it down would allow the player to lean left, right, and forward by moving the mouse in respective directions.
2. Load zones. I think the biggest detractor from the tech demo was the presence of load zones. I often felt a sense of safety because I knew the zone I was in was clear. If everything was open and enemies were intelligent and free-roaming there would be an ever-present dread. Every corner, every dark hallway - no matter how many times you'd passed by them - would never feel safe. There would always be some mystery. I don't know what kind of technical limitations you're up against, but that's definitely something I'd focus on.
3. 5.1 audio. Not a big deal when it comes down to it, but 5.1 channel environmental audio can go a long way to providing an intensely immersive experience. Thief: The Dark Project created a rock solid case for that. It's one of those things that you don't really notice when you're without it, but when you do have it adds a whole different dimension to your perception of the game.
4. Mouse smoothing. Might help to show off that motion blurring! As it is I can only see the motion blurring if I whip the view left to right, back and forth.
4.5 While we're discussing mice it would be nice to have support for buttons 4, 5, and the wheel. There's not a real need for it in the tech demo, but if Overture is to have more features (weapons?) then it's just a thought.
Anyway you're doing a fine job and judging by the screens and info you seem to have things well in hand. Bring on the Overture!