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.
I don't think that's the case. AppleScript is the equivalent of shell scripting for GUI appls.
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.
While never particularly popular, both Apple (AppleScript Studio and subsequent AppleScript Xcode templates) and third parties (FaceSpan) have provided full IDEs for developing applications with AppleScript.
Incidentally, I'd still mostly agree with your comment, as the underlying script runtime isn't really designed with application development in mind. For example, I'm pretty sure Xcode debugging support for AppleScript doesn't even include breakpoints or related features like single-stepping (Apple's Script Editor certainly doesn't), and that Script Debugger had to pretty much implement its own AppleScript runtime to support these features.
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").
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.