Engineers, UX, support staff. There's an entirely separate office in Hudson, NY for security and telephone support for buyers/sellers. Plus rent in Brooklyn isn't cheap (they likely have a fantastic deal for their early office space but DUMBO is an overleased market at this point and they are fitting out new space in "DUMBO Heights").

All the jurisdictions Etsy operates in require various forms of compliance, which you could waste engineer time on or hire dedicated employees to handle. And by going public they've likely had to hire 15-20 people to support their CFO and run compliance.