One of the major differences, from a programming point of view (as opposed to implementation) is that with message passing, you can pass any message to an objec

One of the major differences, from a programming point of view (as opposed to implementation) is that with message passing, you can pass any message to an object, even if that object doesn't know how to handle it. Cocoa makes extensive use of delegates (not sure if these are similar to .net delegates) for handling unrecognised messages, which makes adding custom behaviour to classes possible without subclassing. Rails is (in)famous for making use of Ruby's method_missing to implement all sorts of funky stuff.