A key difference is that in RQ the effect of a spell was described.
So everyone could point to the book and say that Reconstruct did
*such*. As a ref you could fudge it - but the players could see you
squirm to an extent. Another of the things that I expect to be good
about HW is that there are no descriptions of what the effect of a
spell is. So if a player asks to reconstruct what happened and you
say that he sees a shadowy figure he cant point to anything and
complain that he has been short-changed.
So it gives the ref a lot of power.
Of course with great power comes great responsibility.
When it comes to invisibility - why do you decide that the power is
binary. All you have to do is decide its not and presto - problem
solved. You haven't acidentally writen an explanation of the power
anywhere have you ;-)
"You cast the spell and everyone gasps as you fade away. While you
stand still you are hidden by the shadows of the room, but whenever
you move your friends soon spot you and hoot and hollar in derision".