First of all, I want to thank the Pixelmator team for a truly brilliant product. It's fast, simple, and powerful.
Basically the only feature left that I would want in a photo editing tool is local adjustments using brushes (akin to lightroom implementation) and radial/gradient masks.
I know this has been requested once or twice below but just wanted to put my vote on that too.
What features would you like to see in Pixelmator Photo?