How level should a new floor be?
Beauty is in the eye of the beholder.  The larger the room, it easier it is to visually pick up on a level problem.  I tend to be a bit of a fuss budget, but I can usually pick up on a 10 foot span that is much more than 1/4 to 3/8 inch off level. This is a pretty objective observation that would be more pronounced to a trained eye than a casual observer.