When they started using their own, ugly cast lugs and super heavy Matrix rims is about the time they lose me.
I had a '79 900 series. It was beautiful. And my one Trek keeper ... for a while.
I also really liked the '80 412 I had, made of Ishiwata.
My only Japanese keeper right now is an '82 Miyata 912. With the way it's built up (with a chromoly fork), it's one of the best bikes I've ever ridden. It also looks totally badass.
A nicer Fuji, like the Opus, has never crossed my path.