Since I've got nowhere else to put this I'll post it here, feel free to add your own...
1) Put a intensity slider for dirty mode... no-brainer, it can't be hard.
2) I finally figured out what I don't like about Painter's Watercolor system -- it's actually in 2 parts... the first is that they modelled it after a fully transparent dye system(Gel Layer ) instead of a partially opaque pigment system like real watercolors (which makes a big difference, dye makes it more like colored inks) and second there's no "dirty mode" that will allow you to scrub dry color to carry it along with new color into a stroke.
So that said if they can't do a proper pigment style it really limits what can be accomplished in "watercolors" and re-wetting and scrubbing are time honored watercolor staples... all this could be forgiven if they simply added the ability to flood an area with "water" and keep it wet and active until you are done working with it. Without that it is impossible to get a satisfactory wet-on-wet technique.
I've got more to come later.