Pros
The employees and the atmosphere. Having worked at large software companies before, I appreciated Zillow's open policy about everything. Even when the news isn't great news, senior management is very professional about how they discuss things and they welcome an open discussion. Over time, this transparency exposed me to every aspect of managing the business outside of my job description and expectations. This is something I would have never seen from the aforementioned large software company.
Kontras
Restructuring the company to better handle a global slow down in advertisement markets, Zillow recently went through a reduction in force (RIF). This sudden and drastic change greatly affects the company makeup, it's culture, and it unavoidably drains the company of senior talent and "tribal" knowledge. Rapid development and execution of ideas were largely a result of throwing lots of resources at a problem. Rather than rely on the product development team to quickly innovate and then refine an idea, execution of a plan was often bogged down quibling over minor details. Almost making the argument for the RIF, a smaller team with less oversight could deliver a similar, if not quite as polished product, in less time.