Imagine an environment where you can modify and debug every single program you run without ever restarting it; where every little part of GUI of every program is there for you to manipulate (in real time, without restarting) if you want. Environment where you can grab an item from a deeply nested menu and pin it to the desktop, change its color and make it spin, while still being able to click it and invoke the function it's supposed to call. Environment where you can inspect and modify everything in real time (have you seen "Inventing on Principle" by Bret Victor?), from GUI artifacts to underlying models, to system resources, to the very core of the system. An environment which you can clone with one click and run anywhere with a real, not Java-like, guarantee that it will work exactly the same way as for you.
Well, that's still not quite it and there are many other things "left for Smalltalk to accomplish", but hopefully you get the picture.