I'm probably just repeating what some people said above, but what the hell...
- Different types of gameplay mechanics would be a welcome addition. Maybe also increase the difficulty on the puzzles, too.
- Something that dissapointed me from the previews is that when we die in-game we just get a cutscene. It could be nice if deaths were more unpredictable, but I don't know if that's possible within the engine.
- The ability to walk would be nice, although I don't mind too much personally.
- Don't make things easy to players. If we fail over and over, make it harder! That way it will be more rewarding when we succeed. Don't try to appeal to everyone, and make people think with puzzles. Like Ron Gilbert said: "Player be damned!".
- First-person view would be nice too, but not all the time. Maybe in climatic moments, it could add a lot to the atmosphere.
- And now that you have time, record the soundtrack with an orchestra!