Okay, I thought quite much about it, and personally, Id like to see as demolevel either the tutorial of the game, or the small missions seen on the videos (I think they have been vids from E3 presentation, people could vote what they wanted to see next, like room clearing or something), but if they make a real mission from the game as demo level, i wont download it, this is why:
Especially in this (type of) game, its important or at least it adds loads of feeling to the game, when you react on impulse, which is only possible when you dont know what will happen next.
It takes a lot of fun out of the game when you reach the position where you started out in the demo, and play it that way that you played it a million times before when you were waiting for the game. Multiple ways to complete the objective dont help here, the weeks that you will play the 1 or 2 levels in are enough to learn every detail of it by heart.
The best thing that UBI could do, but I dont expect it, would be to make a special level for the demo, that isnt included in the game. Much work, but it would save a lot of fun in the final game.
thanks for reading, agentdennis