Supposedly, Adobe has tightened the language as compared to common browser Javascript. Did you notice in particular how this was done? I'm never learned enough to know what's better or worse.
...different core objects. That is, html is built on top of the Document Object Model (DOM), whereas Adobe uses their own pdf document model. Means that the syntax and basic operators are the same, but the core objects which can be manipulated are different.
The language in and of itself has been pretty good for a while. But if they meant "replacing the browser DOM with our .pdf DOM" then that's not "improving and tightening the language" in my book and they shouldn't tried to sell it as such.