Paintbrush.app isn't quite on part with Paint.net's features, although there was no technical limitation in Cocoa causing this.
Acorn is more featureful, but trying to be more like a mini-Photoshop than an advanced MSPaint. It's very powerful but it trades off simplicity to get it.
I tried my hand at making my own Paint.net clone for Mac OS X 10.4 (Tiger) during my first year of learning Cocoa (2008) but it only ended up being a learning opportunity for me and nothing useful came out of it.
Mostly because I had no motivation. I was sharing my progress in #macdev on freenode but there is absolutely no excitement or culture of encouragement in that channel. And for some reason I feed off other people's excitement, it motivates me tremendously and I get 100x productivity boost. But if there's none (and no paycheck), I drop to 1/100 productivity.
Honestly I'd like to try my hand at it again. I've learned a lot in the past 10 years. But I just don't have the time.
one of the nice things about paint/paint.net is at least partially how limited it is: everything is easy to find, menus are small, actions are limited but quickly understood, etc. It's probably the pinnacle of an 80% product
Paint.net has always being intuitive, an I still remember how to do most operations, even after years without using it. But never managed to run it reliably on Linux under Wine.