One problem I've commonly encountered in OCaml is the inability to expand the interface of functors after they've been created (I'm not talking about post-application). For example, Map.Make in the stdlib takes an OrderedType module which contains only the compare function. What happens if I want to add something to this interface, such as a show function? I have to copy the whole implementation of Map.Make into my own file to modify it. Compare this to the ability to 'include' a regular module and just add the new functionality, and to take the type of a module and expand that type as needed. Functors are severely lacking in this regard.