TextEdit is a simple application for quickly writing text and notes. The text can be edited in RTF (Rich Text Format) mode, as well as plain text. With the following defaults write command you can enable plain text as default format in TextEdit.
Copy and paste the following command in OS X Terminal:
defaults write com.apple.TextEdit RichText -int 0
Revert to the default setting with the following command:
defaults delete com.apple.TextEdit RichText