There are only a few very frustrating things, a few involving characters that hopefully will be balanced eventually.
Mei is so strong, like if you have half a brain Mei is absolutely game changing. I would like to see either her freeze rate lower the time it takes to freeze you or that you shouldn't be able to be damaged by her freeze gun. (Let the spike shot hurt you but not the jet that freezes you).
McCree's flashbang duration is too long (this is the first character being patched by Blizzard).
Bastion, although not even as close to as OP as people cry about should have his sentry time increased a bit, maybe by a second.
The characters overall are extremely well balanced for the most part.
The netcode at times is a big annoying, it goes by "shooters sight." You've probably noticed once or twice you've snuck behind a wall, died and then on the killcam noticed you weren't behind the wall at all.