Okay, well, I immediately know one. @extends in the browser. Right now, we have to do all sorts of gross-dark-magic things to make @extends work in Sass. It's complex and it mostly-works... but it's complex because we don't know the DOM. If we knew the HTML, we could be super efficient about it.
Something with the functionality of @extends would be trivial in the browser, and then we wouldn't have to do it. We only built it because there were so many good use-cases that it overrode our concerns about complexity and maintainability.
Another thing I really want is a good CSS-JS bridge.
Oh, and I'd love it if nesting was supported. We could still compress our output, but we wouldn't have to repeat selectors over and over in our output!