I never used Skitch, but I love this. The controls are simple and intuitive, and the lack of styling options means I won't waste time playing with different fonts, colours, etc. What would make this perfect for me is the ability to paste/copy to your clipboard. Regardless, I'll be using this from now on.
Added and pushed! This idea cuts my workflow way down. You've also showed me that it needs the ability to paste an image from clipboard to the canvas. Adding as an issue.
Love your work. I also just noticed resizing keeps the aspect ratio when you hold shift. Nice touch.