I just have a couple of suggestions, that aren't really all that major I think.
1) I don't really find the transparency all that nice. It's actually frustrating. I can understand the fascination with it, but for me it equates to the HTML blink tag. Fun to implement. but not really helpful in any way.
2) There doesn't seem to be a fast way to set the opacity of the brush you are drawing with, or any way other than layer opacity. Unless I'm missing it? in any case it feels like something that should be up by Brush Width.
Thanks
Dan