They've added Moira. Don't know how much you know about her, but she's quite a game changer in terms of a healer.
Doomfist now exists. Broken af.
Mercy is now completely different which affects gameplay a lot, but its also good because she's not as much of a must pick.
Junk got given another bomb so he now has two which is useful.
Sym/ Zen/ Tracer are largely the same tbh I can't think of anything different between them.
The main differences since Orisa have mainly just been nerfs to characters and the fact the Comp seasons were cut in length by a month so they're a lot shorter now.