Over on the Hubspot blog, Cory Eridon talks about these "16 Things People Really Hate About Your Email Marketing":

1. Emailing People Who Didn't Opt In
2. Making Unsubscribing Difficult  
3. Not Honoring Unsubscribes
4. Writing a Vague Subject Line  
5. Not Indicating How We Know Each Other  
6. Speaking With Forced Sincerity 
7. Providing Irrelevant Content  
8. Bombarding Recipients With Email  
9. Sending Emails Once in a Blue Moon
10. Including Broken Dynamic Content  
11. Not Including a Call-to-Action
12. Sending Emails That Aren't Mobile Optimized
13. You're Overdesigning Your Emails
14. Not Optimized for Plain Text and HTML
15. You're Not Proofing Your Email Before Sending  
16. Not Providing a Real Address to Reply To

Read the full article here (it contains a bunch of examples of what not to do!)