As for the audience you are targeting, "serious writers who want to focus on writing without the hassle of running a website", I see a couple of problems.
First, it doesn't seem like long-form writers have a problem with any of the current Internet publishing platforms. There are some conventional length restrictions for online content that grew out of the length (and style) of reading that's comfortable to do while sitting at a desk. Adapting those conventions to the era of tablets is indeed a problem, and one that's being addressed by things such as iPad magazine apps, Kindle subscriptions and Instapaper. But that seems to be a different problem than the one you're positing.
Second, and more importantly, anyone who is concerned about the "hassle of running a website" won't understand Markdown (or even know what plain text is, for that matter). The concept of a markup language is a hard one for non-technical people to grasp. There's a reason WYSIWYG editors, for all their problems, are ubiquitous.