First off, I love Artrage. I work in database design and everything is very staid, drab and ordinary. So Artrage is a pleasant step away from all this.
I love the simplicity and the feel. And the photoshop export.
Secondly, and this may have been covered by other suggestions, I would love to see a paint spill tool. No idea how it would work. I can see from other comments how to create a similar effect using other tools, but this might be something that could save time.
Keep up the good work. Nah, what I mean is keep up the great work.
Cheers from OZ.