First, you're still using the WordPress API so you have to know that to begin with. If you want to do X, you have to figure out how to do X with WP, then learn Y in Cutlass to do X. You're essentially adding a theme layer on top of an existing theme layer. You save characters, I see that, but I'm not seeing anything particularly compelling on the examples page [1].
The other big problem I see here is maintenance. WordPress is a particularly compelling platform because of its popularity and very large developer community. Build a site in WordPress and it's easy to find someone else to improve and maintain it. Build the theme in Cutlass, however, and now the number of people who can maintain it is drastically reduced. If I saw this in a theme, my first response would be "step 1, let's get this out" (as it was when I saw Smarty being using in a theme), rather than "I should learn this for this one project."
If you develop with WordPress on a regular basis and don't like the theme layer, help the core team improve it, don't write another one on top of the existing one.
Regardless, kudos on the project. It looks like a marked improvement over the normal state of affairs. The built-in templating is far from the worst thing about WP, but it is easy low-hanging fruit to modernize, and I for one am glad to see that effort pay off.
Make it "than" ;)
