It introduced me to the idea that 'is' should be treated very carefully. Any assertion outside of strict formal languages that use it are half-truths at best. It also introduces heightens the emotional tone of a discussion. If you say "John is foo" you tend to create the impression that John will always and has always been foo. Foo-ness is a taint on his soul. Contrast that with reformulations that make it explicit that John's foo-ness is a fleeting association related to both his present situation, your current perception of it and the current socially accepted meaning of foo along with all it's implied baggage.
I realise I might be rather off-topic :-)