Quit Mail before changing plist preferences manually or via Terminal, otherwise Mail will overwrite your changes.
Many UNIX users Prefer their email in plain text, as opposed to HTML or RTF or whatever else is out there. Unfortunately Mail.app doesn't have a checkbox for this, but the preference exists! Edit the mail preference file (com.apple.mail.plist in ~/Library -> Preferences) and add the following lines into the first dictionary:
[robg adds: An alternative way of adding the key is through the Terminal. Just type:
defaults write com.apple.mail PreferPlainText -bool TRUE
Apple Option P (not in the menu system) forces individual messages to display as plain text.