When it comes to the desktop there is Cinnamon 1.8 and its many new features. Additionally, there is MATE 1.6 for reliving the GNOME 2.x experience.
Linux Mint 15 also offers up artwork improvements, ships with Ubuntu 13.04 as its base with the Linux 3.8 kernel, and there's an assortment of other minor changes.
More details on this morning's release of Linux Mint 15 can be found out from the project's blog post.
Linux Mint also offers a new tool called Software Sources, which is developed from scratch and replaces Ubuntu's software-properties-gtk program for managing the distribution -- including derviative distributions. One of the other new tools to Linux Mint 15 is Driver Manager, which is similar to the Ubuntu tool and in fact uses the same back-end and working the same way as software-properties-gtk.