Many people want Emacs to prefix included text with something like ` > ' instead of with three spaces. One way is to change the code of the function `mail-yank-original' in lisp/sendmail.el that prefixes with spaces. A more flexible solution is to use Supercite, which provides wide configurability in how you format included text in replies. See question 107. Both of these solutions work for RMAIL and GNUS.
A related problem is how to prevent Emacs from including various headers of the replied-to message. For this, you should set the value of mail-yank-ignored-headers, which takes a regexp value.