Everybody's making good points about the merits of both styles, but
I've noticed that one thing doesn't seem to have been mentioned-
If you've been following a particular thread, then having the newest
replies at the top makes the most sense, as you've probably already
seen the older stuff. If you're jumping into a particular topic in the
middle of it's life-span, then sure, you might have to do some
scrolling, but so what?
-Simon