It's slightly annoying that you have to twiddle a bunch of the preferences to make it a workable plaintext editor. Apple should just delete its rich text functionality, now that they bundle a real word processor (Pages) with the OS.
In TextEdit -> Preferences..., select the "Plain text" radio button. New documents are created as plain text now.
New Document:
Format:
Plain Text (select)
Options:
Check spelling as you type (disable)
Correct spelling automatically (disable)
Smart copy/paste (disable)
Smart quotes (disable)
Smart dashes (disable)
Text replacement (disable)
Open and Save:
When Opening a File:
Display HTML files as HTML code instead of formatted text (enable)
Display RTF files as RTF code instead of formatted text (enable)
When Saving a File:
Add ".txt" extension to plain text files (disable)
Plain Text File Encoding:
Opening files = Unicode (UTF-8)
Saving files = Unicode (UTF-8)