Jekyll is too complicated. First, it requires a .yml config file, which is unused outside of the Ruby community. So this fails my "you shouldn't have to know what language it is written in requirement". Personally I don't think there should be any config files at all, but if you're going to require a minimal one at least pick something universal and easy to understand, like an rc file or maybe json. Secondly it has the weird _ naming thing. Jekyll was clearly made for Ruby developers, which is fine I just think the world needs something much simpler than that.
YAML is okay when used in a simple manner such as for a basic configuration file, but it has a seriously fucked up spec with all kinds of landmines waiting to blow your legs off. If you aren't generating your YAML with @ruby_object.to_yaml then sooner or later something bad will happen to you.
YAML was crafted to be usable by most mere mortals. Whether or not it achieves that in practice is another issue ... but I think most people who find themselves using a static-site generator could find their way around a YAML config file just fine.