At deeper levels a lot of things are left to the interpretation of the reader, as it is with most abstract things. He uses the word Quality in the most fundamental way. To me that word held the meaning for his life. Quality in the way in which he understands and reacts to the world.
Before he learned to fix his motor cycle, he was at the mercy of various kinds of mechanics with varying skill levels and attitudes. He particularly cites an example of some listening to music while working on the machine. Which he does not like. For him working on the machine or writing technical documents for his job, is a spiritual activity.
Also, he makes a point regarding his nervous breakdown, that for others it was a nervous breakdown. But for him, the way he saw it, it was the deepest of meditations. Following which phase the understanding and meaning of Quality, dawned on him, and he was at peace with his life.
Also, I found the way he treats his 11 year old son, i.e like an adult, very interesting.
Another delightful thing, was the book full of philosophical quotable quotes. Like the (approximate) one I have cited in my above comment.
But most basic reason I liked the book, is because, it talked to me in the tone, I wanted it to talk to me. For example the moment any book becomes very specific (for example 'Life of Pi' is very specific in being inclusive to all religions, in a rather simplistic way), as a reader I tend to start to disagree. But if it is abstract, and "Zen.." is very very abstract in portions, I can provide my own concrete implementations!