Not exactly based on EA org experience, but I think one of the biggest challenges orgs face is going from small enough that everyone can sit at the same table (people sometimes call these 2 pizza teams, because you can feed everyone with two pizzas; in practice the number is somewhere between 8 and 12) to medium (less than 150 people, aka the point at which you can personally know of everyone) to large.

EA orgs are most likely to face the first transition, small to medium. The big thing to know is that you'll have to find ways to take what happened and worked organically with a small team and transition it into processes since you'll no longer be able to easily achieve org-wide alignment automatically. This typically means the introduction of planning processes that make sure everyone is on the same page about what you're trying to do and making sure everyone's work is connected to the org's mission.

Exactly what will work is context dependent, but perhaps the first thing to get right is that you can't expect things to keep working the way they did when you were smaller. There is a tipping point where adding one more person breaks your ability to function the way you used to, and at that point you need to be ready to introduce more formal processes (these don't necessarily have to be extremely formal, just more formal than everyone sitting around the table talking and figuring things out without any process).