Well, the two big ones.
Mercy, right off the bat. She’s had hundreds of thousands of posts written about her since her rework. On the new forums, it numbered 15k last time I checked in the megathread alone. The devs don’t have anywhere close that number of posts read; if they’d been reading the Mercy feedback, the number of posts read would be much higher than they are now.
Two, this post from last October, where Jeff outright admits that the Dev team had no idea how bad Doomfist was.
Three, the QOL changes that Sombra’s been getting; Hack cancelling reload, cancelling translocator, these things have been asked for for years, since Sombra’s release. The bugs in her kit, which have been reported and recorded hundreds of times, many of which have remained unaddressed (documented here)
Four, Bastion’s situation. That really needs no explanation.