Today I came to a fundamental paradox of the object programming style, concrete types or interfaces.

Whats the better election for a method's return type: a concrete type or an interface?

In most cases, I tend to use concrete types as the return type for methods. because I believe that an concrete type is more flexible for further use and exposes more functionality.

The dark side of this: Coupling. The angelic one: A concrete type contains per-se the interface you would going to return initially, and extra functionality.

What's your thumb's rule?

Is there any programming principle for this?

BONUS: This is an example of what I mean http://stackoverflow.com/questions/491375/readonlycollection-or-ienumerable-for-exposing-member-collections