What is different between Mint Linux and Ubuntu other than the interfaces?

Mint actually uses Ubuntu's repositories for most of the packages you'd install. So, outside of the things which Mint specifically overrides in their own repository, Mint is Ubuntu.

You can find what software Mint itself provides here: http://packages.linuxmint.com/list.php?release=Rosa . As you can see, most of those packages are related to the display environment.