Yes I agree, developing dynamic "painting layers" would be a great addition. Allowing interactive smudge type blending on a layer. With settings like 'Viscosity' for oil type behaviour, and 'transparency blend' for Watercolour type effects. Interactive 'Dryness' settings maybe. Now we've moved into Open GL, and looking at what people are doing in Pixel Bender already.
These could be single layers with this feature enabled, a simple control click on the layer and then "convert to Canvas Layer". You could outdo Painter by a long way.
This needless to say could be a new feature in Standard not Extended.
I've owned painter along the way and though I like it's brush affects, and even though they've tried to conform to a standard interface that people are comfortable with, I never totally warmed up to it.
I just thought PS would have updated that by now.