The Cinnamon menus recently received a major overhaul. The old ones were inherited when Cinnamon was first forked from gnome shell, so this has been a long time coming.
The first thing you will probably notice is the absence of the arrow and the resultant gap. These worked well for applets, but looked out of place anywhere else (such as desklet context menus). Instead, the menu now opens right next to the panel, applet, desklet, or whatever else you’re opening it from.
Thanks to Joseph, the animations were also improved, and now look more similar to that of the GTK menus. You can see the animations in action in this video:
The removal of the arrow also allows theme developers to use the full range of CSS properties, hopefully leading to even better themes!
Finally, removing the arrows allowed us to greatly simplify the menu drawing code. This not only makes it easier to maintain, it has also been know to improve performance and responsiveness on some machines.
Note to theme developers and users of third party themes
The bad news is that the arrow was implemented in such a way that we had to break themes when we removed them. The good news is that we were able to do it in a way that doesn’t leave the background invisible, so older themes will not be completely useless. However, doing so required us to switch the style class from “popup-menu” to “menu”. Basically, this means that older themes will still work, but won’t look good until support is added for the new style class. This change also allows themes to maintain support for older versions of Cinnamon, while still supporting the new menus.