Something he didn't say for Genji was after your Q ends and you're sheathing the sword you can Shift out of that animation as well. It greatly increases the speed at which you can start doing damage again.I highly recommend everyone watch this vid:OVERWATCH CANCEL TUTORIAL - YouTube
Guy explains most of the good cancels and is very helpful. Vid posted up yesterday.
Roadhog is around the 9min mark.
The Genji cancels seem required, they make a huge difference. Reaper cancels are easy enough learn to get a little edge and S76 is kinda minor but useful.
Reinhardt/Roadhog cancels seem massive too.
Also with the Roadhog he didn't say that you can RMB, hook, LMB, melee, E and they all cancel each other.