"I appreciate your thoughts on the matter, in fact I encourage you to always challenge me when you feel it’s appropriate. You should never be cowed by authority. Except, of course, in this instance, when I am clearly right and you are clearly wrong."
I like this bit. Dawn has always had a special bond with Willow, way different from how either was with anyone else. Willow was like the cool aunt or cool older cousin, someone just enough removed from the immediate family that they can do no wrong. Except for right now.
Dawn is the first person to call out Willow on this. The looks that Spike and Buffy give Willow tell Willow that they feel the same way, but it is Dawn that gets Willow to change her ways.