Kerbal Space Program

If a configurable field is not public it doesn't get populated on part load.

public abstract class MyAbstractBaseClass : PartModule {
  [KSPField]
  protected float myNonPublicSetting = 0;
}

public class MyChildClass : MyAbstractBaseClass  {
  void OnLoad(ConfigNode node) {
    // Here base.myNonPublicSetting is always 0 regardless to the config settings in the part.
  }
}

When field myNonPublicSetting is declared as public everything works as expected. While it looks a minor issue it's actually a bad one since it breaks OOP concept. Some settings may only be needed by the super classes. Not to mention that most of the config settings are usually read-only for the outer world. In the latter case such settings are usually declared protected/private, and a public/protected read-only field is created as a part of API:

public abstract class MyAbstractBaseClass : PartModule {
  [KSPField]
  private float myNonPublicSetting = 0;

  // An API accessor for the descendants, but not for any caller.
  protected cfgMyNonPublicSetting {
    get { return myNonPublicSetting; }
  }
}

public class MyChildClass : MyAbstractBaseClass  {
  void OnLoad(ConfigNode node) {
    // Here cfgMyNonPublicSetting returns value from the config but cannot be changed by the descendants
    // unless base class explicitly allowed it by declaring setter for the property.
  }
}

I would admit that allowing annotated fields to be private may result in names conflict: super class and the subclass may define a field with the same name, and it will be a challenge to resolve such setups. Well, at least "protected" could be allowed without any side effects since reflection returns both public and protected fields when requesting in flattern mode.