Marketo put together a nice list of items to test in email. I would recommend breaking the elements into ones that impact early engagement metrics (subject line, etc.) – essentially anything that lifts delivery and open rates – and then conversion metrics (offer, body copy) – anything the generates clicks and sales. Enjoy…

1. Subject line
2. From name
3. Day of the week
4. Time of day
5. Frequency
6. Mostly-images vs. mostly-text
7. Short copy vs. long copy
8. Links vs. buttons
9. Number of links
10. Unsubscribe at the top
11. First name personalization — in the subject line
12. First name personalization — in the email body
13. Animated gifs
14. Font colors
15. Font styles
16. Opt-down
17. Social sharing icons
18. Social connecting icons
19. Delivery by time zone
20. Call to action — number
21. Call to action — placement
22. Post-click landing page
23. Social proof
24. Tone — human vs. corporate
25. Copy length