I just ran through chapter 1 a couple times on test.
Main differences,
I encountered no side missions so no more running around forever looking for the objectives, and no Electro frustration.
Electro event no longer occurs, Electro is in the "Southside Power" treasure room.
"Treasure" rooms no longer have any chests, so I don't quite see the point of them other than the one that had Electro in it.
There are also apparently no mission rewards other than XP for any of chapter 1. So no +75 permanent health and no story reward boxes. I hope they make up for that lost HP by adjusting later mission rewards
Overall its more streamlined and surely better for new players.
Trying to find all those side mission objectives and get Electro before he was insta-killed was seriously frustrating in the old chapter 1.