I've been reading way too many Adobe reference documents in my spare time - four books on PostScript and another on the PDF format - add this one on JavaScript. It's nice that Adobe puts all their manuals online but reading them is a mind-numbing exercise. Anyhow, I'm about one-third of the way through the reference you suppplied.
The JavaScript language itself is pretty plan-jane standards compliant from a syntax point of view. The difference lies in the [link|http://partners.adobe.com/asn/developer/pdfs/tn/5186AcroJS.pdf|Adobe JavaScript Object Specification]. What I haven't figured out at this point is: (a). does the javascript run in a sandbox? and (b). does filling out the forms on a client require you to purchase Acrobat - or can you just use the free reader?