It was so good! It's too bad it never became more popular and disappeared.
As an avid HyperCard user one of the problems was you couldn't extend it with new UI components. I did write an XCMD (C code to extend HyperCard) but there was no way to visualise new stuff, I understand it was because of the highly optimised code by Bill Atkinson. The extensibility is probably why Visual Basic took off.
Ah, the good old days.
WindowScript was a popular XCMD that would let you open multiple windows with native controls (which is how many stacks added color support long before HyperCard did).
And inheritance too! Every card inherits from its background. It was basically OOP.
I'd suggest reading up on Self -- or Smalltalk-72 for that matter.
My first OO programming language was the 'MOO' language used in the online LambdaMOO text-'VR' worlds.
They used to have a really perplexing pricing model, but I think you can use it for home now mostly unrestricted.