Quill's off to a good start with what looks like a very nice vector painting engine and nice, gradual response on the drawing trigger, but I immediately noticed that there were some unused controls. Right-handed configuration here, invert for left-handed as necessary.
-The left trigger does nothing. I had thought it was a second tool, but it's just unresponsive.
-The left analog stick also does nothing when the grip trigger is released, considering the lack of tool functionality on that hand.
I figure these things could be used for more shortcuts, and I'll get to that in a moment.
-Undo is mapped to X.
This isn't a problem in and of itself, except that I can't seem to find a redo to go with it - and given that I had just launched Medium a moment ago, I was expecting it to be a left flick on the left stick. (Redo is a right flick.) Also, Medium uses the left trigger for hot-swapping your right-hand tool to the Smooth tool when held; you may want to consider a custom tool-swap if the left hand continues to be the menu/support hand and not a tool hand.
The menu systems are very different between both, so I don't expect things like using left stick up to show the tool palette or left stick down to show the layer menu, especially as when the menu is open, you can just tab left and right with the left stick, no poking with your right hand necessary.
Much of my interface concern is the potential muscle memory issues arising from lack of standardization between Medium and Quill input-wise.
I know they're made by different teams for different purposes (Sculptris/ZBrush modeling on one hand, SAI/CSP/Mischief painting on the other hand), but imagine if you had to use a different undo shortcut because nobody agreed on Ctrl-Z across the board, or the usual cut-copy-paste with the Ctrl-X/C/V cluster. That just makes for unnecessary confusion.
Think of it this way: both teams are effectively laying the groundwork for productivity software in VR, and as such, it would be a good idea to agree on where the most common functions applicable to both should be located on the Touch controllers, particularly things like undo/redo and layers. At least both programs already agreed on mapping brush size to the right-hand stick.