Sometimes when I read a story, on the story's own page (linked to from the main headlines page), it says there are zero comments. Even after 2, 3 or more hours. No way have /. members slowed down that much! I can set the threshold to zero and still see no comments.
In all such cases there's a yellow box stating "This discussion was created for logged-in users only." I am already logged in to slashdot. There's a "Log out" link in the upper right. That's enough to convince me I'm logged in, but apparently the site thinks I'm not.
Alternatively, perhaps this message is written in some misleading way, misrepresenting the reason comments are not shown.