At this point, this is nothing more than another "me too" sort of post, but I like it.
I can agree that there is probably more whitespace than needs to be, but I don't find it distracting.
I'm fairly indifferent on "position: fixed" elements on a page, so I can't relate to the out-rage over the header and sidebar.
I can copy & paste into text boxes again, so that's a plus.
As for performance, if there's any kind of "comment auto-loading" based on scrollbar event firing, see Learning from Twitter
by John Resig. I haven't personally experienced this yet, so this may not be the issue at all.
Anyway, definitely an improvement in appearance.