A simple syntax check would be nice ... like the PHP one, as mentioned before - and why not an inline check as implemented for PHP in v9.x?! Sure, Webdeveloper Toolbar or Firebug support these features, but imho Firebug really slows Firefox down if it is enabled all the time... Not that nice working environment...
And the JS Console is always one click away - definitely it's no number one must have feature, but it would be nice - why not?!
The framework support is really difficult - the frameworks are always changing and somebody has to update the libraries for the auto-completion. Who wants to do this, especially for which framework?! There are so many out there - ok some of them are more popular then others - but nevertheless somebody has to overview the changes...
Maybe the DEV-Team implements an interface, so that people from the community can implement such specific features

But imho I think it's better to develop / enhance the application itself - just think of storing files to FTP (with storing process in the background - so that the application doesn't freeze, if you have connection problems); but that's another topic
Just my two cents, EvilBMP