Octopress is designed to remove all the time-consuming and frustrating tasks you usually have to deal with when using Jekyll. You don't have to write your own HTML templates or do any configuration to get started. The default template also takes care of any basic CSS/JS you need to write to get...
Middleman is a big piece of software, it's not simply a static blog generator. Because of all the functionality and flexibility it offers it can be a little more complex than other static site generators and a little harder to learn all of its bells and whistles.
Handlebars' philosophy of "no logic in templates" makes some things difficult: DocPad built-in template helpers aren't available by default - they have to be manually added/exposed DocPad's example template code often includes logic, which makes it impossible to use wi...
Nanoc is a good choice if you need to support a site with a more complex structure than a simple blog. Nanoc is more agnostic to the types of pages you have, and allows you to do finer tuned refinements like customizing the URL structure.
Hexo has a relatively large following and community, especially in China. While this is certainly a positive, many developers who do not know chinese would be unable to follow all the guides and tutorials out there written by their chineses counterparts.
Because metalsmith is at its core focused on transforming directories of files, it can be used for more than just static site generation, and could be used as a build tool, a documentation generator, or any use that requires file transformations.
The Metalsmith community is still fairly small compared to the more popular options. This results in a lack of learning resources and difficulty finding support from experienced users. However, a slack group has recently been started.