I would argue the Smalltalk-esque message-passing system is an advantage, not a weakness. A large amount of the dynamic nature of languages like MacRuby can be

I would argue the Smalltalk-esque message-passing system is an advantage, not a weakness. A large amount of the dynamic nature of languages like MacRuby can be captured with such a system whereas on .NET you need another abstraction layer (like the Dynamic Language Runtime), leaks and all. And I would nitpick the argument that Objective-C requires types for arguments. You can declare arguments as "id" in Objective-C, exercising message passing, and objects will respond to the messages regardless of type. This is why MacRuby and Objective-C are a good fit: both languages are based messages, a concept alien to .NET.