If you can see Overwatch's Genji you've probably let him get too close. The same applies to his character in Heroes of the Storm. And if that happens, whatever you do, don't shoot him.
A quick highlight posted to the game's subreddit shows exactly what happens if you decide to ignore that wisdom. At level 20, Genji's Deflect ability can block incoming damage and also return the favour in the form of thrown Kunai blades that target each of his opponents. There's also a Reflect talent players can pick up on the hero to make his Deflect ability even more deadly. Hence how you can obliterate an entire team simply because one trigger-happy Tracer decided to go rogue and start firing right after Genji tapped "W".
With his X-Strike ultimate waiting in the wings as well the other team really didn't stand a chance.
Overwatch characters started taking over Heroes of the Storm in autumn of 2016 when Tracer was first added. Whether you think that's been an awesome development or has somehow made the game less unique, getting to see Tracer/Genji fights recreated on an isometric battlefield is great.