Well apart from the Unity and that both are very customizable, the most notable differences are:

MENU

The Administration and Preferences menus contain several options not found by default in Ubuntu like:

MintAssistant – Allows the user to set a root password.

MintBackup – Backs up the home directory to a single file.

MintDesktop – Allows the user to select desktop icons, specify the Nautilus mode and determine whether or not to show splash screens for major applications.

MintNanny – Child protection.

THEME

It is "More Green" than Ubuntu ;) Like more "hulkier" Me like Green kind of green.

PACKAGE MANAGER / UPDATE MANAGER

In Ubuntu you have the Software Center we have seen so far when using Ubuntu since 10.10. In Linux Mint they have their Software Center which is similar but includes the option to run .mint files.

SCHEDULE RELEASE

In Ubuntu if they say Ubuntu XX.XX will be release on the 10 of October at 10am with 15 minutes and 20 seconds, you can be sure it will be 30 seconds after what they said but it will still be that day ;)

Linux Mint does not yet have a so perfect schedule release. They have a "When Ready Release" or the famous "Soon".

DRIVERS

While Debian does not have a easy option to install proprietary drivers and Ubuntu gives you the option to install it, Linux Mint installs them automatically, as well as additional proprietary softwares like Flash and others. In Ubuntu 11.04 for example you have the option while installing it to include those drivers (And also the video ones like Nvidia).

This is just in the top of my head but there are MANY MANY more like the amount of devs in Ubuntu and the amount in Linux Mint. Where the code is based in Linux Mint (Github for example) and the one in Ubuntu and so on...

Hope this helps.