They seriously streamlined the item stats, removing lots of the super specific affixes that made finding gear that worked for you a huge pain in the ass. They’ve also added ways to control some of those affixes through Masterworking, which lets you roll for one or two of the affixes from specific categories.

They’ve also changed how paragon works. Now max level is 60, and relatively easily reached, and then you begin earning paragon levels that are now shared across your characters on the realm (eternal, seasonal, hardcore). This way you can get your full build in terms of skills pretty quickly, and then become more powerful through the paragon board.

With I think the fourth season they added The Pit, tiered timed dungeons like in D3. With the fifth they added “Infernal Hordes,” a vaguely roguelite dungeon where you adjust the enemy mechanics as you go through waves until the boss fight, trying to earn currency to open chests at the end.

The expansion added another dungeon type called Kurast Undercity, which starts with a certain amount of time and you gotta kill stuff to get more time. These have customizable rewards based on how you start them.

They also added a thing, Dark Citadel I think it’s called, that requires multiple players and boss fights have mechanics that require you to work together. It’s fun, but I prefer to play single player.

Helltides are always active somewhere now, and in all difficulty levels. Farming them is pretty great, and a fast way to level up.

Getting materials to summon the endgame bosses has also been streamlined, so they’re easier to farm.

Also the new class, Spiritborn, is stupid OP right now, which is always fun.

Oh! And you no longer see mobs levels, to help the people who don’t like that enemies level with you and say it ruins the RPG part. Now enemies are just enemies. They’re always going to pose a similar challenge except they generally get easier to kill as you level up because of your gear and build.

I’m sure I’m missing stuff, but that’s some of the improvements they’ve made.