I played D3 pretty thoroughly for a month or two after it came out, but became dissatisfied with the player auction house. It always seemed like it was easier/faster/less grindy to buy something from another player than to actually pull it from destiny's mangled corpse. Which, considering that the D series is all about leaving behind a trail of mangled corpses, kind of killed the purpose of the game.
What did they change?
Got rid of the auction house, for one. Completely revamped the loot drop system and the balance of gear enchantments, so health regen and life on hit actually does something, and stats are huge. When I came back to playing, the old gear I had at lvl60 had stats of like, +50, compared to new gear drops with stats of +200. The "smart loot" system makes it so if you're playing a wizard, most gear drops will be for wizards, having intelligence, vitality, and other key aspects for wizards. This applies to all classes, so there's much less chance of getting useless gear drops for a character, like a barbarian getting a legendary barbarian item with intelligence on it.
Legendaries and set items drop much more frequently. Especially when you're playing on harder difficulties (called Torment, ranging from Torment 1 to Torment 6 in intensity). No longer do you need to constantly farm Act bosses like D2. Just kill enough critters on hard difficulties and rng will give you something eventually.
The Reaper of Souls expansion really added to replayability by giving a "sandbox" mode made for just killing and looting without worrying about campaign progress. And the addition of seasons and regular new content releases with each season provides incentive to come back and play again to see what's new.
Balance is overall just way way way better too.
It's a lot different than when it first released.