Ability to change or replace object instances (maybe with instances of different types) on the fly could be an example. In general, you can design incredibly dynamic and reconfigurable systems. Whether you want to or not, and at what level should you stop doing it, is certainly a good debate to have.

If the system you are building makes sense, there is probably a reasonable type system that shows it to be well typed.

Yeah, "makes sense" and "reasonable" would be criteria involved in the debated I mentioned.