His biggest mistake was probably including the police and the firefighters in his plans. They are well-respected members of Ohio society and have powerful, well-organised unions. But even so, the scale of his defeat suggests that even had these two groups been excluded (as Scott Walker did in Wisconsin), he would have very likely been in trouble. In America they call this sort of politics "overreach". Where I come from it is called "biting off more than you can chew".
There is no particular love of unions in Ohio. One old woman, with a shotgun propped up nearby, told a canvasser "get off my porch you union crony". Moderates said things like, "I'm not a union person but..." and basically explained in one way or another that they didn't think the new law was fair.
There was a real show of union strength, with people coming in from all over the country to organise and campaign and someone even told the crowd the morning before the results came in that they were going to stuff Kasich's law down his throat. I was told that there was an incredible "energy" among the unions that had not been seen in a long time. This energy is now heading for Wisconsin in an attempt to unseat the governor there, who has enacted similar legislation. Beyond that, they will be working on the 2012 election.