I think a good balance for a markdown syntax is something you can write nearly naturally on the fly (very low syntax needed for basic use) but can continue to mark up in more detail when you want to turn something into a complex use case (very high syntax ceiling for the complex cases). HTML+CSS almost gets there except the tags are a bit awkward for simple things like just <strong>bolding</strong> a word.

Regarding Notepad, keep in mind they dumped an entire separate app (WordPad) by merging the ability to format into Notepad. This kind of mirrors what I was talking about above: one app with a very low floor but a flexibly high ceiling when you want to go there with it rather than two separate apps you need to pick from up front.