I'd never thought of this before. I especially like your examples with the delimiter for switching between syntaxes. CoffeeScript provides one of those (http://jashkenas.github.com/coffee-script/#embedded), and I think you're right that it'd be nice to have.

> I find: ({} a b c d) -> js: {a: 'b', c: 'd'} Better than the single { equivalent.

([ 1 2 3) and ({ a 1 b 2) are kind of an abuse of s-expressions. I think the fact that I want them suggests I should be going all the way to [1 2 3] and {a 1 b 2}, without the parens at all. If you have to delimit them with parens, I agree [] and {} have advantages.

(Not to mention that it would screw up all editors :p)

Haha, yes it would. I finally had to turn paredit off in emacs when I was writing these expressions.