This project represents a stark tradeoff: lose some complexity, and lose almost all control. It feels like forcing users to learn a new language is a step too far here - I can't imagine a ton of people are so eager to build slackbots that they're willing to give up all their existing tools.
It feels like HN regular are unlikely to be the ideal audience for this. Reaching for a metaphor, it's like a bunch of expert Unix users gathering around an IBM PC or Macintosh in the 1980s. It's so easy to bemoan the control we lose (a major problem given our existing workflows) and have no use for the functionality it confers (simplicity, batteries included, lack of yaks that need shaving). A really great new idea…
This ain't exactly new, though. The value proposition here is very similar to that of a lot of existing cloud-hostable middleware products, e.g. Dell Boomi, albeit perhaps targeted at a different audience (that is: adapted to actually building backend systems instead of tying a bunch of existing systems together).
Boomi, however, is a flaming pile of trash (and I say this as someone whose dayjob is - among numerous other things - to maintain a bunch of Boomi processes); the version control sucks (and you have to use its version control, because what's Git or Mercurial or Fossil or even CVS, amirite?), it's way overpriced, it's a pain in the ass to do simple things with it (unless there's already a shape defined for the exact thing you're trying to do, or it happens to be something conducive to JS or Groovy scripting; the JS/Groovy escape hatch is the one good thing about Boomi), and you're permanently locked into it unless you want to completely rewrite everything in a different language and for a different platform.
If Dark can address those four problems - namely, by supporting external version control systems (Git at the minimum), having a sane price point (and not gouge users on connector licenses), making it easy to do easy things and reasonable to do hard things, and providing some pathway to escape vendor lock-in (e.g. import/export from/to a self-hosted option, preferably an open-source one) - then I could see myself at least trying it out. As it stands, though, there's no indication that Dark doesn't have the same exact crippling problems Boomi has (made worse by forcing a specific new language instead of relying on existing ones).