1. Spamming.

The Internet has a long and deeply entrenched resistance to unsolicited, commercial email. Sending your newsletter without getting permission in advance is a no-no, regardless of how valuable your content is. Rule of thumb: You may send one sample copy, clearly marked as such, explaining how the reader can subscribe.

2. Typos and misspellings, poor proofreading.

Trust me, you cannot proof your own work effectively, nor can you rely on a spell checker, and no one can proof well from a screen. Get someone else to check your work from a print out.
Idea: swap proofing with a colleague.

3. False pretenses.

I see a lot of email newsletters that masquerade as ordinary, non-commercial email messages. This irritating to the reader, who quickly ascertains that your message is really a promotional piece and wonders why you won't just come out and admit it.

4. Timid presentation.

This mistake sometimes leads to the false pretenses error above. If you are going to send an email newsletter, treat it like a newsletter. Give it a name, a table of contents (if you have more than a couple of screens of text), and let your readers know what they are receiving in the subject line.

5. Weak writing.

Get, read and use The Elements of Style by William Strunk Jr. and E.B. White. This 92 page book will invigorate your prose, and if you purchase it from this link to Amazon.com I will receive a 15% commission. Thanks!

6. They are hard to read.

Use symbols such as the slash, tilde (~), and asterisk to offset titles or sections of text. Set your email window size to no more than 80 characters ( a setting of 60 is often recommended) so that text will break into easily readable lines. Break long passages of text into short paragraphs. Break different topics into titled sections and give your readers a succinct table of contents at the top.

7. Subscribe and unsubscribe information is missing or hard to find.

Make it easy for people to whom your newsletter may have been forwarded to subscribe.

8. Intellectual property rights are not protected.

Protect your own intellectual property by dating each issue and including a copyright line:

© Molly Gordon, 1999. All rights reserved.
Or   Copyright Molly Gordon, 1999. All rights reserved.

(c) is not a legally recognized symbol for copyright, so it is best not to rely on it.

Protect the intellectual property rights of others by getting permission before using their materials in your newsletter. Brief quotes are generally permitted, provided you give attribution.

9. Failure to build subscribers.

If you want to build your subscriber base, make it easy for people to feel good about forwarding your newsletter. Encourage them to do so by granting explicit permission (as long as they leave your copyright line intact) for them to send the newsletter to others.

Make it easy for people to subscribe AND to unsubscribe. The latter is important because, on the Internet, people want assurance that they will not be spammed.

10. People pleasing/No personality.

Some newsletters fail because they try to appeal to everyone. Better to show up BIG in your newsletter, attracting the people who will thrive through their contact with you naturally than to attempt to reach everyone and nourish no one.

BONUS
11. Failure to post a privacy policy.

This is worth emphasis. Internet subscribers are jealous of their privacy. If you care about that, say so, say it clearly, and say it in every issue.