Today Jakob Nielsen posted an article about the first 2 words in a link on webpages. His researched showed that many people can guess the content of good links within 11 characters. While that doesn’t mean that we should now limit all links to that length, I think there is a good hint in the results. Make most of what is important understandable quickly.
From reading many articles about e-mail usability lately I think these results can also give good tipps for e-mail subject lines. If you want people to understand your e-mail in an overview list quickly, create subject lines that can communicate the meaning in a quick way. You can still add additional info in the subject line, but have the first words explain the important part.