Honestly almost nothing haha Some character and gameplay tweaks here and there (Like the biggest one is changing teams from 6 players to 5).
I never played heaps of the original Overwatch, so it's still a good excuse to get into it again. But otherwise, it's like the same game. Most of the maps are the same from the original. I think there are 3 new characters?
And there's a season pass instead of loot boxes. Although I will say, I think the season pass is very stingy. There's a gold coin currency, and the avg cosmetic item costs around the 1200 - 2000 mark in gold coins (Which is like $20+ AUD). But the only way to earn that currency is by completing the 3 weekly challenge, which adds up to only 60 gold coins total. So that's something like TWENTY WEEKS if you wanted to buy a single cosmetic through gameplay alone.
I think it's atrocious TBH. Almost every other season pass model is less stingy than that.