Mail.app deleted my subject column!
Like most who prefer a more readable and efficient mail view, you may have used WideMail (or LetterBox) before Snow Leopard. I did, and apparently the Snow Leopard ripped into my plugins and mauled the subject column. I restored it by deleting a couple nodes from: ~/Library/Preferences/com.apple.mail.plist #1 Quit Mail.app #2 double click the plist file and remove: Root/ActiveViewers Root/TableColumns #3 Save the plist and start Mail.app Subject column is now present, done.