There’s a [full list of changes on their blog](http://www.pixelmator.com/blog/)[^note], but my favorite is the new alignment guides:
> Building precisely aligned image compositions is now short work in Pixelmator 2.1, with the new smart Alignment Guides. This new feature lets users position objects with pinpoint accuracy and build compositions quickly by using individual objects or image elements. The Alignment Guides gracefully appear and disappear onscreen as they are needed, while Object Spacing automatically shows when objects are equally spaced on the canvas.
[^note]: Post link is an affiliate link.