Mint has better colors.
No, seriously : just try it. You'll understand why many Mint users (who have tried Ubuntu before) like to point out they moved. The "classic" Mint is as accessible as Ubuntu without all the fancy crap (and better colors). It has a very slick UI, excellent configuration tools. The Debian edition of Mint (LMDE) is also rock solid and very easy to use for a newcomer (I mean, for a Debian distro).
Anything about Mint shows how serious and skillful they are at respecting their users.
IMHO, the reason you hear so much about Ubuntu is the great amount of Windows users who switched because some guy told them it was so cool and simple to install, and while that's true in many ways, they still don't know anything about linux and end up having a lot of problems using it. Or they just can't stand the colors and are getting nervous.
Note that I'm not a Mint user : I'm running Gentoo on my office laptop, and Debian on servers, so I'm not doing PR here.