I actually like the dark gray on light grey for collapsed comments - it provides a great visual indicator that it is there, allows a preview of the comment, but the muted color combination makes it more subtle and less prominent than higher contrast combinations would.
Now, that said, I do agree with all the "too much whitespace" comments, which certainly apply to collapsed comments as well - there is too much whitespace surrounding the text in each collapsed comment item. A collapsed comment doesn't have to be a huge bubble, just make the light gray background slightly larger than the contained text.
A text issue, unrelated to color scheme, it would be nice to display an ellipsis or some visual indicator showing that the collapsed comment does indeed have more than the one line available in the preview. It's a little thing, but I get annoyed when I click to expand a comment and there isn't any more content beyond what I read in the preview.
Finally, the light grey collapsed comment bars should NOT extend out the full width of the page. Keep the green story bars and displayed comment bars going the full width, but keep the collapsed comments visually within their parent comment (perhaps 80% of the parent comment space?).