What follows is an approach to software development that is not appropriate in all setting. Clearly startups are already acting like a start up so they are not the target audience. Neither are large, complex enterprise projects, e.g ActingLikeAStart would not be suitable for managing the software implementation for something like Obamacare.
For the purposes of this discussion lets consider a project requiring 1 development unit. A development unit will have a dev team, a QA team, a PM, some Business analysts, some management, a few other miscellaneous bodies, and a couple of execs sniffing around. The project will have 10 – 30 heads and take between 6 and 18 months. Not a massive undertaking, but definitely not trivial. The project will involve new development, will often require integration with legacy or partner systems and possibly data migration.