What makes code maintainable?

I read Do best practices really matter? a while ago, in which the author argues that what truly makes code maintainable is a set of qualities.

Here is the list of those qualities:

1. readable:
   • the code is easy to read and understand
   • it's easy to see what it's doing and why
2. discoverable:
   • it's easy to understand how code is supposed to be used in common cases
   • e.g. most users don't want to read through a massive man curl page to figure out how to make a simple HTTP request
3. predictable:
   • it's easy to understand what the code will do in a given situation
   • e.g. there are no unexpected side effects
4. limited blast radius:
   • it's easy to understand the impact of a change
   • e.g. if I change this line of code, what else will happen?
5. debuggable:
   • it's easy to understand what went wrong when something goes wrong
6. testable:
   • it's easy to write tests for the code
   • this includes:
     • being able to mock out dependencies
     • not having to set up a lot of state to test a small piece of code