I appreciate the work that must have gone into this, and it looks like it has been done well technically. However: How many people are this deep in the world of Ruby and yet also enjoy writing WordPress themes? I run a forum devoted to WordPress, and I can not imagine any of the experts (who post there) wanting to switch over to Haml or Coffeescript. Most people who use WordPress are either designers or programmers who are looking to put up a somewhat quick-n-dirty CMS.
If you have the skills and resources to install a Ruby environment and a bunch of gems, why not use Rails plus some CMS built with Rails?
I can only present anecdotal evidence here but I do most of my freelance web dev work in Rails but turn to WordPress for web sites. I'm definitely more conversant in Ruby than PHP, but I think the ecosystem of the WordPress platform, plugins, and developer mindshare is a big deal for the projects' futures.
That said, I don't see myself using Wordless. Aside from my personal gripes with HAML, I have some concerns about breaking WordPress and PHP conventions. Will future developers be able to continue my work? Will plugins work as expected? How about page templates?
I did use SASS for a couple of themes and that worked out well enough, but I doubt I'll use it for future projects. Idiomatic code and tools are a bigger productivity boost both for myself and others.
EDIT: I can see a sweet spot for this in teams that work primarily with Ruby/Rails but want to leverage WordPress on a site they maintain in the long term.
If you have the skills and resources to install a Ruby environment and a bunch of gems, why not use Rails plus some CMS built with Rails?