Here's my weird-but-yet-interesting idea of the day...
In most mailing lists (including non-geeky ones), HTML mail is usually discouraged but the newbies still want to make use of bold and italics and the rest.
So, here's my simple solution:
Allow the user to use a rich text box to get all their HTML-coolness but instead of sending raw HTML, send across the text formatted in Markdown syntax.
The advantage is that the recipient actually has a choice of seeing a plain text view or a HTML fancy view. This also encourages "clean mails". Power-users will like this feature as well.
P.S. Of course, Markdown is not the only choice of semantic text styles, there are many choices such as reST, Textile and the plethora of wiki syntaxes. I just find Markdown to be the simplest and most readable of the ones I've used.
I write all my posts using Markdown and it has never gotten in my way of writing and yet works well. That says a lot about its good design.