There have been a suprising ammount of changes made to the game since Diablo 3. Almost enough to qualify it as a new game. Spawns have been changed, boss battles upgraded (few suprises there) and loot also seems to have taken a turn for the better. Sadly rendering a lot of imported stuff obsolete.
No longer do you have to play the game three times either. The difficulty adjusters have been changed, and nightmare,inferno etc. seem to be gone.
The new class is a new twist on things making going through the story again more interesting if you already did that with every character.
TDLR . New loot , new monsters, continuing story, everything that makes Diablo great !