What is a Reiki Master?
Technically, a Reiki Master is a person who has received all of the necessary initiations for each level of Reiki, the 4 Reiki symbols (including the knowledge of their usage), and the training and knowledge that goes along with each degree of Reiki.
For me, however, being a Reiki Master means much more than this. In short, a Reiki Master is someone who uses Reiki daily, someone who tries to live by the Reiki Principles, someone who has learned and continues to practice the techniques that s/he was taught, someone who allows themselves to be guided by Reiki, someone who lives and breathes Reiki.
This is quite different from the "technical" definition of a Reiki Master. A Reiki Master is a person, i.e. a human being, just like everyone else. S/he knows this as well, and doesn't present him/herself as being better than anyone else. A Reiki Master is a person who has dedicated an enormous part of his/her life to using, being in relationship with, and ultimately living Reiki.
I believe a Reiki practitioner needs "seasoning" to fully grasp the concepts at the Master level. Without plenty of experience, the major point will be missed. I think one of the best ways to express this idea comes from a quote in the book Dharma Paths by Ven. Khenpo Karthar Rinpoche. This is a book on Buddhism and not Reiki. However, the concept of having enough experience and openness to realize the higher teachings of any given system is described very well. Here is the quote, "This level of teaching is secret, not because there is some kind of partiality or exclusiveness, but because a given practitioner many not have sufficient openness and intelligence to receive it. For example, because human beings have language, we can communicate with each other more completely than animals. Because we have greater intelligence, we can understand and learn from each other more effectively than animals. We have not somehow hidden our language and intelligence from the animals, but no matter how much we might want to communicate our concepts to them, they are unable to comprehend them. Just as our concepts are incomprehensible to the animals, the vajrayana is self-secret to some people." (Note: Vajrayana is a particular path or sect in Buddhism.)
In the above quote, the phrase "self-secret" is the key phrase. Just as one must have a solid foundation in addition and subtraction before being able to do multiplication and division, one must have a solid foundation in Reiki 1 and Reiki 2 with adequate experience to fully appreciate the teachings at the Master level. We could try to teach children multiplication and division before addition and subtraction, but they would not be able to understand the concepts.
It is for this reason that the Master level is traditionally taught as an apprenticeship that takes at least one year to complete.