I can't hope to compete with Discourse on features (yet!), but I think Flarum's elegance sets it apart. Out of the box, it's pretty, fast, easy to use, and works beautifully on mobile. And it's written in PHP, so is super easy to install – any low-end shared host will do.
This means that the software will only be usable from interactive sessions on "standard" mainstream browsers, and won't be easy to integrate with batch-processing tools (like web scrappers or data warehousing) or non-standard environments (anything enabled for the semantic web or developers creating handcrafted personal scripts to enhance their workflows). Classic "static" forums don't suffer from such limitations, being based on simple html and having predictable navigation structures.
I believe we are long past that idea, it's just too much of a restriction for the web in order to compete for audience with other technologies (meaning apps).
a.k.a 99.99% of the target audience. I'm sure syndication tools could be easily built into it to provide static views to scrapers etc.
Everything is "fluid" and flat (as in hierarchy flat). I like the more traditional hierarchical type of forum.
On top of that, with Discourse, you're never too sure if/when there is a new message and if you didn't miss anything.