In languages like Markdown and Textile, one doesn't need to worry about presentation, it's all about semantics.
What I'm saying is that untrained users have trouble with the idea, fundamental to markup languages, that some text can represent attributes of other text. To such users, an asterisk means "the text has an asterisk in it." They cannot reinterpret asterisks as markup without relearning the way they look at text itself. They're used to having the territory laid out in front of them, and all of a sudden you're handing them one map and asking them to write another, but they don't know what a map is.
<p><b>some text</b></p>
<style type="text/css">
.emphasized { font-weight: bold; }
</style>
<!-- [...] -->
<p class="emphasized">some text</p>