I've only been playing around for a little bit, so there may be ways to do some of these things. Even so, these are things that have kept me from really diving in, so as far as new user acquisition goes, it might be something to think about.
1) Paint bucket style tool or canvas color option! Brushes are great and all, but sometimes you need a solid layer underneath.
2) Brush size is a paint to deal with - that things seem to cap at 25px, then you have to discover that you need to click it again and drag it past... changing the brush size just seems a bit of a pain.
3) Something equivalent to sponge / dodge / burn. I know this isn't photoshop, but this is a pretty useful tool in illustration. It speaks to a larger issue which is essentially that you have to get what you're going for the first time - it is unclear how to modify things already on the canvas other that undoing, modifying the parameters, and redoing.
What would be really nice, though probably quite difficult to implement, is if strokes were selectable and able to be modified.