Rehearsal clips are teasers, meant to feed our appetite without giving too much away. Like a good movie trailer they should pick our interest without giving away any major plot twists in advance.
Many of the blogs reporting from the rehearsals fall into the trap that I myself have fallen so many times when on location: you start paying too much attention to small details - what gorgeous lighting, what a cool backdrop, what an odd camera angle - and forget the big picture. If your song isn't strong enough in the first place, few things can help you. It can happen, but it is fairly unusual.
So these reviews are written before I have seen anything from Rotterdam (on Monday night to be precise) and here are my views before anything else began to blur my judgment.