I agree with Johannes - would be useful if it could extend DLTK or JSDT to:
- parse annotations
- show the jsdoc for each method/class etc when a token is hovered over, just as Eclipse does for java
- support the ctrl-space code completion
- support the ctrl-click to drill-down
- implement "show call hierarchy" as per java
- show errors and warnings that would be detected by the compiler
Currently the "Closure IDE" seems to be not so much a development environment, but a build tool with a bit of colouring for soy templates.