I once made a suggestion about blending tools, but I'm not sure they would be as useful, now that I've used Artrage more.

However, here's my wish list:

Movable/Re-scaling layers! (Would do wonders to improve the composition!)
The ability to mirror and flip layers (Goes hand in hand with the previous point, really)
Adjustable brightness/contrast for individual layers and selection areas (Again, that would let us experiment more!)
Selection tools, specially the lasso (And if we could also save/retrieve selection areas... Oh joy!!)
A more agile hide/unhide layers process (There's a noticeable delay in AR2, while PS does that instantly, by contrast)

Also, I'm not sure I like how the color picker (I mean the tool with the drop, not the thingamabob at the bottom right) works currently: I can rarely get the same color twice when using it

And I can see the intensity of the colors degrade when switching back and forth between PS and AR2

That would be just about it!