As a web developer and someone who programs in a superset of HyperCard (LiveCode) - I must say it was moderately difficult to grasp in the beginning. Strangely enough things that are required for your day job tend to have a way of getting learned. The hardest part for me was not the syntax, but the fact the IDE and the language are so inter-connected.
It's actually a fairly quick way of prototyping mobile apps for iOS/Android - which is one of the main things I use it for at work. Though, I am probably incredibly biased.
LiveCode is a vast superset of the former HyperCard, and I work with it everyday. It's extremely utilitarian, and it's not half bad when prototyping mobile apps.
* Yes
* Community Version is open source
* On the LiveCode roadmap
* Compiles as a native app for iOS,Android,Mac,Win and soon HTML 5
* No, but ... the ability to wrap other languages and libraries and map them to HyperTalk is in development: http://livecode.com/blog/2014/07/08/the-next-generation-widg...
* Definitely as fun as the original
Sorry if I am coming off as a fan boy, but I read HN everyday, and this is the first time I've seen HyperCard/HyperTalk come up.