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"What are the chances in NYC that I go into a cafe and meet a startup founder?"As someone who lives in SF and is moving to NYC, your odds are pretty low. But I think there's a better question here:
What are the chances in SF that I go into a cafe and meet a startup founder working on something big and impactful?
I don't know about you, but my time in SF has netted me a lot of opportunities to talk to founders who are working on [insert social/mobile/local thing here] which IMO simply aren't big enough (or original enough) for me to care. For every Square there are dozens of Square copycats who aren't really executing anything substantially different. For every Zynga there are dozens of startups in SF egging their exact formula and not really mutating it in a meaningful way.
Certainly very, very few are working on anything that has relevance outside of the tech community, with impact IRL - I'm talking about the AirBnbs and the Squares. Stuff that will change the lives of people who aren't plugged into the pulse of the California tech scene.
SF is full of startups, sure, but the vast majority aren't doing anything interesting. I'll trade quantity for quality, though I'm not asserting where one might find higher quality between SF and NYC.