Fun. For Everyone.
That’s the most basic way I can describe it; your job is to be a transmission vector for fun. You have to take everything you know players will try and do, including the stupid game breaking bullshit, and work out how to tell a story that threads through all that to ensure everyone has a good time. The combat bunnies, the plot obsessives, the quiet ones, all of them. If you can solve those problems, or at least solve them enough, that everyone has fun? Job done.
And if you can push people’s boundaries a little while you’re there and maybe introduce them to people and concepts they’d never considered before? So much the better.