I'd argue one of the most pressing concerns right now is the lack of migration tools
Currently you can't just create an account on instance X and move to Y. You need to create a new account. Eventually if we get the functionality to migrate from one place to another, people will be able to spread out across the fediverse and the risk of a single big server going belly up reduced.
From a technical standpoint if one instance gets defederated from other instances, all the users on that instance are stuffed. Their content won't appear in the wider fediverse (so less engagement)