Discourse founder here. I tried their demo forum but it currently isn't responding. Probably too much load from HN although their home page claims they can handle 1k posts per second (trollface).
From a cursory point of view, it seems to have more in common with Disqus than Discourse. It's embeddable, doesn't allow rich posts or markdown or our support for embedding videos, github, wikipedia, amazon, etc.
It's also closed source, although it is free and they claim they'll never insert ads.
I have no idea about their support for the other challenges we've faced such as infinite scrolling that is compatible with the back button.
I am skeptical of redis as their only data store. We use redis too in Discourse but this would demand that all forum content can fit in memory at once, and there is a chance if a server crashes you'd lose so. Not to mention querying it is much more difficult especially when filtering.
Either way, we're glad to have the competition!