The Ramban provides us a basis to postulate an answer to this question. He says that from the time of the covenant with Noach, Hashem's retribution for sin will be similar to the rainbow, which can only appear if there is at least one clear piece of sky. Hence, the rainbow is miraculous in the sense that it serves as the paradigmatic archetype for the change in the nature of Hashem's relationship to mankind. An alternate answer to the first question, which in the process answers the second, can be found in a teaching of Rav Shimshon Raphael Hirsch. He writes that the rainbow, light refracted into bands of different color, is representative of all different types of people, ranging across the spectrum of holiness.
All humans come from the same source.
It was only after the flood that people diversified across an array of levels and expressions. Assuming the essence of the rainbow represents the state of mankind, it was a miracle that a rainbow could exist before the flood, when such a division among people did not exist. It is thus an appropriate sign for all humanity of Hashems promise not to destroy them, en totum. In the Torah it is written that during the generation of the flood every living creature had corrupted its ways. An example of this corruption and the ultimate cause for the flood was the lack of respect which individuals accorded the money and private property of others.