Lean is built with Bootstrap 3 and its responsive grid. Your site will properly resize to the device that it is being viewed on.
The entire theme was built with HTML5 in mind. The markup was designed with SEO in mind so you don't need to. There's also a SEO admin widget to input your meta description and gives you a preview of what it will look like on Google. If you use another SEO plugin, it will let that plugin do it's thing instead.
Lean supports the Standard, Status, Image, Link, Quote, Video, Aside and Gallery post formats. Who doesn't like variety, right?
All of the CSS files are built with SASS and then compiled and minified. All of the colors can easily be modified in the _variables.scss file. Only the necessary styles from Bootstrap and Font Awesome are included, but excluded styles can easily be added.
The entire theme is built with Grunt. There are tasks for compiling your SASS and JavaScript. All third-party libraries are pulled in using Bower and only used during compile time. You can also view the documentation using Jekyll.
There are a number of custom page templates that come with Lean. There is an Image Slider (the current page you're viewing), Jumbotron, Sitemap, Archives, Full Width (without sidebar) and Default (with sidebar).