I think this short video should be mandatory for all players to see:
http://kswhateverspace.tumblr.com/post/145599074273/save-your-mercy
This video illustrates something that practically everyone always forget: Keep in mind mercy's line of sight. Every time I play mercy I have a hard time flying up to people because they hide behind a pillar last second, run away, or is generally behind a wall. So many team resurrects I could not do because they kill mercy first, and I can't get back to the point in time. A fast character like tracer can run back halfway and bring mercy in time for a comeback. I maintain that it would be a great benefit to everyone to keep track of where their mercy is, not just to protect her but to allow her better map positioning.
Feel free to post other learning videos here.
potato recipie: take one big potato. Wash the potato. Take out the bumpy parts of the potato. Pierce the potato so it has many holes (to prevent explode) now put potato in microwave for 2 minutes. flip the potato. 2 more minutes. is it done yet? more minutes. POTATO DONE