Regarding newlines: in Emacs 23, I recommend visual-line-mode over auto-fill-mode, unless you actually need hard line breaks. For when you do want hard newlines, (setq-default fill-column 80). I never understood why the default is 72; it makes text files excessively narrow.
The default is 72, in my understanding, because of the historical use of email.
As folks successively quote each other in a thread, the prepending of '>' to the quoted text requires something narrower than 80, else it'll hit the 80-chars-wide limit and wrap.
72 would allow 4 or 8 quote-levels, depending on if your client decides to insert spaces around the '>'.
