JavaScript's key event listeners, but those are a complete and utter pain.
Canvas is also somewhat nice once you figure out how to work with it. I still don't completely understand the ideas behind it. There seems to be a specific paradigm behind it that none of the tutorials I've gone through have shown me.
The fact that it is so fucking hard to respond to keystrokes and write some pixels on the screen in modern programming languages is more than a little damning.
Canvas is also somewhat nice once you figure out how to work with it. I still don't completely understand the ideas behind it. There seems to be a specific paradigm behind it that none of the tutorials I've gone through have shown me.