one thing I love about Smalltalk is being able to inspect the code as it's running. Especially from the GUI. You can just be like Oh I wonder where the code for this button is from, inspect it, and it takes you right to the code. It's a shame we can't have this level of introspection without negative performance implications.
We could in Java Swing (and Visual basic, but lets not talk about that) as well. Well, not runtime, but we could design applications and just right click on the objects in the IDE and go directly to the click (or double click or right click) handlers.
Then again, not having to disable this for runtime is a feature if you ask me.
Yes, current web applications are prettier and easier to upgrade but we lost a lot on the way wrt developer experience and ux.