There's almost no learning curve if you know Photoshop versions 6.0 - CS6. From this point of view, Pinta has a bit of a learning curve, or maybe there's some bugs, or maybe it doesn't like my system colors. It also seems (not sure) to lack common brush and tool settings, although maybe it draws differently than PS.

Simple easy UI and enough capabilities to accomplish the occasional manual image editing tasks I need as a dev, in a free and open source tool.