How Mission critical is your project?
When you evaluate projects to work on, particularly if it is not a personal project, it helps to consider giving a lot of importance to how 'Mission critical' the project is. You might be developing in the same technology stack, but there is usually a magnitude of difference between what you learn while working on a Mission critical project versus what you learn in a normal project. All the required stuff that goes into developing a system that just cannot go down will be very crucial for you in the long run.
File this in the same folder as A New Metric For Startup Job Postings. The parameters that developers look for when choosing an assignment is fast changing. Number of users is just one - Mission criticality is another - we will see more!