I’m quitting my PhD. The decision has been made. The reasons boil down to diminished interest in the research subject and disappointment with the university’s managerial practices (nothing is done on time or well). A while ago I informed my supervisor about these concerns and gave him the heads up that I may be leaving after a period of self reflection. He found my concerns justified and made reasonable adjustments, what was within his power, but it finally rained down on me that no matter what I won’t pull forward the PhD.
We have an amiable professional relation and I’m sure he may be interested in knowing what went wrong and when. I also want to diminish the blow as I suspect he had put lots of hope on my research, not to mention that I was his first PhD student and my departure will be a hindrance to his tenure. He also has a bit of an ego which I'd rather keep appeased (This is in Europe, in one of the countries with more hierarchy-based academic systems.)
In hindsight, what concrete information would an experienced academician expect to get from a PhD who is leaving?