Wintersmith has an extensive Markdown support. Default, it is rendered by Marked. However there are plugins available for others (such as Markdown-it).
The documentation for Wintersmith is lacking examples and clear explanations. It's also quite disorganized, making it difficult to find what you're looking for.
DocPad's has a large amount of plugins available to extend its functionality and compatibility with other language preprocessors and markup languages.
Javascript preprocessors include: Coffescript, TypeScript, and LiveScript.
CSS preprocessors include: LESS, SASS, Stylus, and Roole
HTML markups include: Markdown, and Textile
Templating engines include: Eco, Handlebars, Moustache, HAML, CoffeeKup, Jade, and Teacup
JSON converters include: YAML and CSON
DocPad provides a LOT of extensibility and dynamic capability, which means there's more up-front investment to learn DocPad well -- and deviating from the defaults while maintaining project robustness may be difficult.
Hexo built sites can be easily deployed to Github pages, Heroku, Openshift (custom cartridge needs to be setup) or any other custom solution (just copy over thepublic folder). Any deployment is as simple as editing the _config.yml file and running the hexo deploycommand
Hexo has a relatively large following and community, especially in China. While this is certainly a positive, many of the Hexo guides and tutorials are written in Chinese.
The entire Metalsmith core is currently ~350 lines of code in total, and the system is built around the simple idea of manipulating a dictionary-like set of file objects. This architecture makes writing plugins and understanding the Metalsmith code base super-easy.
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 API for writing plugins for Metalsmith is dead-simple. If you're already familiar with Node.js you can learn everything you need to know to write a plugin in about a minute of reading.
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.
Assemble makes it easy to work with layouts. Layouts are used to "wrap" pages with common page elements, such as a header, footer etc. You can even nest layouts!