One really neat feature from the now-defunct dailypaul.com was you could click on those vertical lines to the left of the comment (one per level-of-indent), and it would take you up to the comment that started that vertical line! Very helpful in navigating around.
Also, for the "comment header" to indicate the user being replied to, would be helpful.
Definitely agree with the whitespace; the comment I'm reading and this text box I'm typing in are 304 pixels wide, where this browser is 1920 pixels wide! That's 84% whitespace! Quite a waste.
And, this comment might go to "another page" making it far less likely to be read by many (pretty sure you'll take the extra click to read it).