Mercy need to be buff, she's pretty useless at healing and has no survivability compare to Lucio, what they need to do is to change the way Guardian Angel works, put a bigger cooldown to it but make it heal her when she's using it, each time she's flying towards a partner gives her X healing points... could be great.
Mercy has the strongest solo healing of any of the support in the game, with the ability to buff her target's damage by a not small amount, has the ability to bring everyone back to life around her(that comes up VERY often if you have any damage sponges on your team like roadhog), and recovers her own health if you just stop taking damage.
I dunno, maybe the hundred or so hours of the TF2 medic(which plays almost the same minus the escape button she has built in) taught me how to survive effectively. Be very good at dodging shots, always look behind you, and don't feel afraid to pull out your gun if you feel you need to(solo'd a soldier 76 the other day who misses his rocket opening salvo).
Ultimately, both her and Lucio are different enough to be great ideas on Blizz's part. I've lost games with a Lucio because a Lucio can't pump out the solid single target heals that a Mercy can, and I've lost games because we've had a Mercy that couldn't pump out the steady AoE heals that a Lucio can. There's also the difference between Res making people around you alive after a stun, and a Lucio preventing any death in the first place with his res. Mercy gives a slight advantage in her healing power, but Lucio has a bigger advantage in helping his team kill shit around him(always funny that some forget his AoE pushback does damage). They're just really well balanced.
Zenyatta and Symmetra are different matters.
