Pros
I was apart of the Seattle digital products team which was very much like a start up (as of 2014/2015 anyway). A like most start ups, it's a relatively small compared to other companies in the area like Amazon and Microsoft. You'll really get to the know the people here, and they're generally great people you can grab a beer with after work. Over the course of the year I was there, the culture evolved into something of a fun and inclusive work environment that encouraged everyone to participate in ping pong tournaments and nerf gun fights.
Cons
Like I said in the pros, this is very much like a start up. While it may be exciting at times to see how things are moving so fast, the quality of products often take a hit as a result. As a designer straight from college, I'm looking to hone and learn how to adapt design processes to fit each particular design problem I'll encounter, and I felt that the culture here still needed a ways to grow in terms of emphasizing and executing process. Too often, I felt we made assumptions without testing them, developed features without consideration of the outcomes, and made decisions without the proper stakeholders present. I believe the culture here is very much, "just get the job done," often requiring me to perform tasks outside of my job description.