I absolutely adored the atmosphere and sniping mechanics in the first two games, but then the gameplay was basically just "walk to next sniping point, shoot 30 dudes in a row who all know you're there" and that was super lame. It seems like they are trying to do better than that this time, but true open area nonlinear gameplay is something extremely few have done at all let alone done well, so I'll have to see the actual game.
For this demo, I know they intentionally had the helo exit, but I really hope the whole base doesn't just instantly know where you are after you kill the big guy. If you can kill their snipers and they don't hear/pinpoint your location, they shouldn't be able to when you kill their leader.