I would also say that the ability to customise the toolbar, etc. is essential, as well as being able to define keyboard shortcuts, pleeeeese....
As for the actual 'painting' end of it... Something like DCTV Paint's 'Flow' control, and various painting 'modes'. I still can't get away from DCTV Paint, no matter how old/quirky it is...
I know other paint programs have similar modes, but they never seemed as intuitive to me....
:-P
