Hacker News tends to generate interesting, long discussions, which is great. I read the first screenful, glance at the bottom of the window and see that comments reach all the way to the bottom, so I hit the spacebar to bring up another page and keep reading, and the process repeats.
However, on the last page, the comments still reach all the way to the bottom of the screen, and I wind up hitting the spacebar a couple of times before I figure out I've actually come to the end of the page rather than my browser being sluggish or whatever.
If there were an inch or so of the white page background below the last comment, I think that would work wonderfully.
A footer would do as well.