Loren McDonald lists 10 email marketing mistakes in his Email Insider column:

1. Making it difficult to unsubscribe.
2. No "welcome" message and/or waiting weeks to send the first message.
3. Overmailing.
4. Using a large single image as the core of your email.
5. Not using alt tags.
6. Relying on graphical links.
7. Not having a preference center.
8. Not designing for the preview pane.
9. Using a person's name in the "from" line.
10. Hiding email registration.

Read the full article here.