List Building Secret #1 - Make Your Content Relevant Keeps Your Subscribers Happy And Boosts The Amount Of People Who Will Recommend Your Newsletter

One of the biggest problems I see newsletter and ezine publishers having online is that they write about subjects no one in their market is interested in. They just write about whatever they think people want to know about without doing any of the research, which is a huge no no.
Another problem is, publishers who have a market well defined don't then write the content in their newsletters that interests their readers. There is no use having an ezine on coin collecting and running articles on stamp collecting just because you couldn't be bothered finding the right content for your ezine.
This is easily overcome. Just send out an email asking your readers what they want to read. Also sign up for other ezines in your market and see what they are writing about.