Yeah, I know the web hosting landscape has changed since WordPress took off, but if the goal of Discourse was to be the WordPress of forums, using Ruby and Postgres instead of PHP/MySQL seems like a big misstep.
WordPress (and most traditional forum scripts like phpBB) work fine in these situations. Just upload a few files, set up a database and run the installer to get the thing working.
Discourse doesn't. It also generally isn't integrated into one click install software like Softaculous or Fantastico. This means that the audience using a typical shared host like Hostgator and clicking a button in their CPanel to set up their site can't easily use Discourse like they could WordPress.
So yeah, that's an issue here too. Discourse just requires too much control over your server for most shared hosting accounts and can't easily be integrated into one click install setups/setup by merely uploading a bunch of files. So the mass market won't ever touch it.
I'm honestly not a huge fan of Jeff Atwoods, and that started long before he started SO. To me it just feels like he's a bit too idealistic in his approach to things.