Text ezines versus HTML format is a highly debatable subject. At this time, most ezine publishers stick with text, and I strongly recommend you follow suit. Many people are still using email clients that cannot read HTML messages - others prefer speed, ease and security of receiving their ezines in text format. It is safer to use format that is compatible with largest number of users.
But there are other readers who appreciate design quality and visual appeal of an HTML newsletter. So what can you do to please both sides? You can always publish a text version and include a link to an HTML version online. This is what I do and it seems to make majority of my subscribers happy. It does call for twice amount of work since you'll have to make two versions of your ezine, but in my opinion it's well worth extra effort.
So what is a text ezine? Basically, it's just an email with some special formatting applied to make sure it's readable by everyone, no matter what email client they may be using. You'll want to stick to a fixed width font like courier or Monaco, and you'll need a plain text editor like Notepad or Textpad. One drawback with Notepad is it has no built-in spell checker, so I'd advise you to download Textpad or something similar. Many free text editors can be found at http://www.Download.com.
You'll need to set up a template for your ezine so its look will be consistent, issue after issue. This should include your ezine name, date, and issue number at very top, followed by a table of contents. If you're not sure on how it should look, you can obtain some free ezine templates by sending mailto:firstname.lastname@example.org .
At very end of your newsletter template you'll want to include a paragraph on how to subscribe and unsubscribe, copyright information, and advertising information. It's vital that you include unsubscribe information in each and every issue. Do not hold people "hostage" by making it next to impossible to get off your list, it won't be appreciated and is not only considered unprofessional, but it could get you into real trouble with Spamcop.