For me it was the gradual devaluation of Elite status. Removal of Xmas gift, then poorly marketed as offset by some donation to schools. Change in threshold gifts, amongst others. I didn't see any point upgrading to an outdated business class cabin.
I was Elite x2 (bank year) and decided to split my flying with Qantas / One World.
Although meals can be a hit or miss on all airlines, on average, I got more of my favourite meal, a beef fillet that was not overcooked on QF.
When I stepped into the Sydney First Lounge for the first time, my alliance was switched! Reciprocal access to First/Platinum One World or partner lounges around the world, meant inflight meals wasn't as important. I now see Star Alliance as a poor cousin to One World.