What I recommend is to get familiar with the major forum / message board packages as an end user. Seeing as you're posting here, you are probably already familiar with at least one or two of them. I've noticed that they generally use a type of pseudo-HTML written with brackets instead of <tags>. It takes a little acclimatization, but after a short while the tags are as natural as HTML.

I suggest using this system because:

1. It works.
2. Your users will probably already be familiar with a similar system.
3. You will have complete control over the tags, including the ability to create custom tags that combine multiple HTML elements.

Here are some examples of existing systems:

• PHPBB's
• vBulletin's