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This has always surprised me, since I learned it.

What are the features of Smalltalk that allowed this to happen? Conversely, what is stopping this from existing in more modern dynamic languages?

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Smalltalk has simple and strong reflective features. Moreover, it does not make a difference between the developed program and IDE. This means that doing things like that are very natural and well established in the Smalltalk cultural background.

Indeed. Having the whole system in front of you, and knowing how patterns like MVC or Thing-Model-View-Editor encapsulate parts of it, makes it very easy to "reason about" wholesale changes to the system.

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