Even if the don't collapse, It might be nice to see first level comments have some kind of distinguishing visual context...to get to another thought... eg, comment number 2,3 etc...is ofetn buried under 1.1.1.1.1.1.1.4 sub comment discusion minutia....when a top comment has 30 or 40 sub comments.
you should be able to scroll a page and pick out the top 5 original comments...even without collapsing...