Smart, conscientious coworkers and an employer that treats us well
Pros
My coworkers are ridiculously smart and competent. Every day I learn something new from one of them. We move fast and think about having impact on the world. Our site is used by a billion people. The company cares about its employees' quality of life; we have excellent food, shuttles to/from San Francisco, a culture that eschews meetings, great ergonomics, onsite medical care, and a gym. New engineers go through "bootcamp," which exposes you to all aspects of the codebase in a friendly environment where you're allowed to ask n00bish questions and bond with other new engineers.
Cons
It's difficult to come up with meaningful downsides to working here. We move very fast, so I don't always have time to be a perfectionist (but in general that's a good thing). Our codebase is huge and it can be difficult to find documentation.