Primitive vs Abstraction when designing library API

Question

In this video after approx 35 minutes Krzysztof Cwalina (Micrsoft's .NET Framework Program Manager) talks about distinguishing library, abstraction and primitive types in their design process.

Examples of each:

Library types are at top layer: Diagnostic.Debug, EventLog

Primitive types are at bottom, have minimal interface and supposed to transfer information between components: System.String, Int32

Abstraction types: ICollection

When to expose some type through interface or pack behaviour in some primitive data container (or both)?

Think of BCL was designed in such a way that System.String would be an interface.

interface IString : IEnumerable<char> {
  char this[int index];
  ...
}

In this case other API would accept IString and not System.String

void Format(this IString message, params object[] args) {...}

So whether there should be an interface you your library or not is a stuff to think about. There may be exposed both interface and type (string and IString). But Microsoft choosed to expose only the System.String (and no IString).