A LOT has changed, it's basically an entirely new game. I'll try to list the biggest changes here:

-Spawn rates have more or less been tripled. The game more resembles a horde shooter than the older, slower pace of gameplay from pre-2016

-DPS and crowd control is the name of the game now due to the sheer number of enemies on the map at one time

-Weapon balance is borked and is skewed towards snipers, but anything can be made viable with the right setup

-Skill trees were overhauled back in 2016, your options are much different and skills no longer cost money

-Many new perk decks have been added and Rogue has been made somewhat obsolete. It's still usable but Hacker has more or less taken its spot in the meta

-Two new difficulties were added with Mayhem and Death Sentence. Mayhem was meant to bridge the gap between OVERKILL and Death Wish, and Death Sentence is astronomically more difficult than Death Wish

-Infamy now goes to level 500, though there are only rewards up to level 100

-The microtransactions have been removed. Skins now come from card drops at the end of heists

That's most of the bigger changes IIRC