Haven't wrapped my brain completely around this yet, but I wonder if it's going to allow for some cool (and/or dangerous?) crossover write-once-run-in-browser-or-locally apps.
You don't write applications in AppleScript; you use AppleScript to control other applications in an automated way.
You can write AppleScripts with simple dialog boxes and such, just as you can write interactive bash scripts. And while you could theoretically implement an application entirely as a shell script or batch file or AppleScript, it'd be a bit like mowing your lawn with a pair of scissors.
On another side note, while Yosemite is the first time Apple has supplied or supported JavaScript as an OSA language, third parties have integrated the Mozilla JavaScript engine into OSA in the past in what I believe is an abandoned project (Google "JavaScript OSA").