First off, the app now boasts a new interface inspired by the look and feel of the latest major version of Apples desktop operating system. As noted by the Pixelmator Team:
Pixelmator 3.3 Limestone has been redesigned with an all-new look and feel inspired by OS X Yosemite. It keeps the black interface Pixelmator fans love, but enhances it with translucent elements to cohere with OS X Yosemites fresh and modern look, all while maintaining the power and speed users expect from Pixelmator.
The latest update to the app also boasts full support for the new features and technologies built into OS X Yosemite. These include iCloud Drive integration for saving and accessing image files across Apple devices, and extension support for using Pixelmators repair tool in other apps.
Theres also support for Continuity, particularly Handoff, allowing you to work seamlessly between Pixelmator for Mac and Pixelmator for iPad.