Mainly that Blizzard did an amazing job adapting everything to a controller, from the way spells are targeted to the hub-based interface. You would never know from playing that this was a game originally designed for a mouse-and-keyboard. And of course, sitting back with a controller is way more comfortable.
But also, it’s how it feels like a different game when you have direct agency over the character’s movements. Diablo on a PC – most top-down action RPGs on a PC? – always felt like you were pulling the dude along on a string. But with a controller, I’m moving the dude directly instead of telling him where to stand.
-Tom