People wanting to be there for the sake of the place instead of just a specific job. Specific jobs and roles can pull people anywhere.
Can there only ever be one Florence at a time? London, New York City, etc. are definitely world class but only San Francisico dominates the cultural narrative. This criteria makes the requirements more strict but ‘feels’ more like what we mean. There are many cultural narratives however, so the definition of what dominates a cultural narrative may change.
The “groundbreaking things” piece is important because World-class cities do not necessarily have world-class scenes. Additionally, this is neither necessary nor sufficient for a Florence because scenes can be undiscovered (though maybe less so with the internet).