CEO-founder Michael Lebowitz said Big Spaceship is fast because it's "flat" and not organized around the traditional cascade approach, where strategy flows to production to design to technology.
I'm seeing this first hand. A lot of digital agencies have mimicked the structure of their mainline agencies. Clients want to be close to the work, to the humans actually doing the production, not separated by 3 layers of acct structure in the name of "service". If you're straight with the client that they're not going to the "service" they're used to, they'll be fine. Most are willing to trade some ass-kissing for some more dollars to put into the program.