I think most of the amazement came from the time they were in. In that time this was truly revolutionary. Having a whole OS based on it is still arguably revolutionary, but from a programming/IDE perspective I don't think they are anything special any more. As far as I know a modern Lisp + Emacs gives you roughly the same experience.
Not sure what you mean by "live editing of inline data structures" here (care to give an example?), but the interactive debugger in Emacs/SLIME can do quite a lot of nice things, including modifying arbitrary data on the fly, live.
For instance, if I have a hashtable returned from a function I called in REPL, I can inspect it and modify its values and properties. Also, within the REPL itself, text is "smart" and copy-paste tracks references, so I can paste the "unreadable objects" (i.e. #<Foo 0xCAFECAFE>) directly into REPL calls and have it work, because SLIME will track the reference linked to a particular piece of text output.
The presentation based REPL in Emacs + SLIME was inspired by the Symbolics Lisp Machine presentation feature.
But I can assure you, there is a difference of a REPL feature in an editor and a GUI using it system wide, as on the Lisp Machine. Both in depth of the features, integration and the feel of the user interface.
Check out this video (which I made some time ago), which shows the presentation UI from an application perspective (here a document authoring system) and as a bonus, the application integrates Zmacs (the Emacs of the Lisp Machine)...
Think of the Documentation Examiner a version of Emacs Info. Think of Concordia as a version of an Emacs buffer editing documentation records. The listener a version of the Slime listener. You can also a short glimpse of the graphics editor, IIRC.
