The multitouch trackpad support means you can now take advantage of pinch-to-zoom, two-finger swipe to scroll, and three-finger swipe to navigate your browser history. Smooth scrolling is now enabled by default. Most notably, Opera has decided to make the browser compatible with older Macs. Unlike many competitors, this version will work on legacy PowerPC machines. The browser will also run on OS X 10.4 Tiger.
The development process took so long that bug-fixes were implemented beyond those originally planned for version 10.50, which is why the browser is now at 10.52. The full changelog for Opera 10.52 can be read here.