Like in Maya when editing a numerical value directly rather than by way of a slider the value remains highlighted after it's edited. Makes for a much faster way of experimenting with different values.
Ability to merge layers. I get that this makes it difficult to remain non destructive, but I would be ok with that if I had the option to merge layers. This reduces memory load as well as clutter.
Ability to bounce texture to project shelf. Rather than needing to export the texture and re-import.
Save Colors/Color Wheel. A color wheel would be awesome, but even more so would be the ability to save a slew of swatches inside a project or even a shelf
Selection marquee and lasso
Ability to transform and hide meshes
More mask by selection type options. Kind of like the polygroups options in Zmodeler to be able to create and edit object ID maps inside of painter.
Better lazy mouse. (IE to be able to see where the mouse tail is. The dark circle around the brush tip is horrible.
Better alpha in the viewport. The current alpha often is either all opaque or all transparent. Or only shows the object that has partial transparency behind the object it is in front of. IT would be nice if this worked better.
Increase the handling of high res models. Many times my models are too high to load in substance even though they aren't using very much of my computer resources. This means that I'm losing detail because I need to work from a normal map from ZBrush.
That's all I can think of now. Thanks!
Please split this in multiple feature requests, so we know what people are voting for. (Also check that some of them are not already exsiting ;)