For me the moment the X-Men jumped the shark was the moment it went from being the X-Men to being the X-Universe, and that was 1982 with the dual publication of the New Mutants graphic novel and the Wolverine mini-series, which is ironic because they are both good stories in of themselves, the problem is they diluted the concept and the focus of the X-Men.

When X-Men was a single book (not the core of a franchise) there was a singular vision guiding it, a singular story being told, and an inherit limit to the size and scale of the storytelling that prevented it from becoming a bloated mess.It was the story of the X-Men, a small group of misfits fighting the good fight in a world that feared them. With those two publications it became the story of a group of misfits fighting the good fight in a world that feared them and the story of the next generation training to take their spots and the story of a noble savage-the berserker with the heart of gold, and from there it kept adding ands to the story as a splash effect form the shark jump. Once it jumped and made the big splash, the ripples kept going outwards and the focus, the purity of the concept and the quality became diluted, more so over time as the ripple effect got bigger and bigger. But the actual shark jump was when the book stopped being a book and tried to become something else-a line, a franchise, a cash cow, or whatever Marvel tried to make it. There was no going back. Part of what made it what it was, was the fact it was a book where anything and more importantly everything happened. After 1982, everything didn't happen there. It was no longer one story being told, one concept, etc., etc. It was something different. (Some may argue better, but that doesn't invalidate it being different).

-M

I'm with badwolf: the resurrection of Jean Grey and the subsequent creation of X-Factor was the turning point, for me anyway.