While I'm sure most people would not advocate it generally, writing HTML by hand is painstaking and laborious. To illustrate, let's consider the two ways you can create HTML - manually in a text editor or via a WYSIWYG editor. To clarify my original statement: HTML does not work well for the casual user quickly composing notes.
But for writing notes, it is less than ideal to put it mildly. I love HTML and it's truly a pillar of the modern world. I hope this will be enlightening in that regard. While the overwhelming majority of feedback on the new editor has been positive, I think it's not completely clear to everyone why we went with Markdown as the underlying format. I started composing a response and wound up with a rather lengthy discussion on the drawbacks of using HTML to write notes and the benefits of Markdown which I decided to convert into this blog post.
This was justifiably met with some rather civilized disagreement as being inaccurate and overly broad. HTML was never intended as a medium for composing text in and the fundamental design of it has severe limitations that make it extremely clumsy to adapt to that purpose. data-markdown - use markdown, sometimes, in your HTML.