It's not just the fact that there's a Grunt file with it, it's the way that the Grunt tasks configurations are organised that makes it unique. One file per task, and project structure organised as variables means that much of the individual task modules are re-usable as they are, without further configuration, in forked themes and plugins.

* Want to know what browser versions autoprefixing accounts for? Look in grunt/config/autoprefixer.js.
* Want to know what Bower dependencies are copied into the assets directory? See grunt/config/bowercopy.js.
* Want to configure PHP Mess Detector settings? Look in grunt/config/phpmd.js.

All easy to find and get to grips with, and all potentially re-usable. The individual configs are loaded in as needed, and all tasks are loaded just-in-time.

(Disclaimer, I wrote most of the Grunt config for Flagship).

Rob Neu has done an amazing job with the Sass arrangement too, particularly with the variables. "Private" and "scoped" variables make it easy to take an existing version of Compass, and change the values / variable names to amend colours or typography and rebuild the Sass to have a theme that looks very visually different. I'm using most of his setup (with permission) as the basis of my own starter child theme for the Genesis Framework.