I've not touched the new entity.module helpers much, in particular the $operation related handling. I'm no fan of the $operation term, while I think introducing the concept makes sense. It's the analogy the view-modes for entity-forms and if field API would support it, it would help a lot to ease customizing registration forms or configuring different form-widgets for searching? I think this needs some more though and probably a rename as it doesn't map to operation (that would be e.g 'delete', 'create'?). Anyway, given that this blocks some further important work I guess we can care about that in a follow-up.
To be completed.