That Unreal Engine Demo was really impressive!
I really liked how it benefitted from the engine in terms of effects, but didn't automatically look like an Unreal Game with the caracteristic bump-map effect etc. Like Mirrors Edge, it kind of wrestled the engine into putting out somewhat unique visuals.
The Voice System for Coop simply blew me away. Did the shark actually react to the call? This is really innovative stuff. If you pull this off, you should advertise it as one of the game's main features.
But here is some critisism:
I bought the game before it exists yet. Only to watch an insanely impressive Video of a game that i will never get to play. The Unreal Demo feels lightyears more capable and advanced than all the Unity Stuff. I feel that IM would be a WAY better game if it used the Unreal Engine 3.
It's even more frustrating now that the UDK was released for free. If i may be so bold, i think you should return once again to the Unreal Engine 3. I think that Engine is much, much more capable to realize your vision than the Unity Engine.