The book Blood, Sweat, and Pixels, the chapter on the calamity that was D2R Items launch recounts the way a Blizzard player played for literally hundreds of hours until they finally found one piece legendary loot. The moment that the orange light appeared out of the shadow of an enemy, he went to the treasure only to discover that his character could not even utilize it. The loot system was inherently broken that the thrill of working for hours which was then followed by relief of receiving something unique, was broken.
It was then fixed which you could only get certain types of loot which would fit your class, and the frequency at which early-game famous items would be dropped up. This meant that even though the legendary items you received did not break the game, you still felt you'd occasionally be hit with a tiny amount of dopamine and keep you in the game.
In the event that Diablo 4 gets that right and uses a similar loot system similar to Loot2.0 which was introduced in Diablo 3, then we're already worried about just the amount of time we'll get to spend playing the game. Diablo 3's debacle is the best possible thing to happen to the series on a continual basis. If you pair this together with the Immortal controversy, it seems like Blizzard is able to provide a straightforward strategy for avoiding potholes for the sake of remaining among the best of it's the most faithful players.
The community is vocal about what it doesn't like, and has been throughout the lifespan of diablo II resurrected items, so we're hoping Blizzard will be taking this feedback from players fortunate enough to experience large portions of the game prior to its release. We'd wager that Blizzard doesn't like the tidal wave of leaked footage that has been surfacing on the heels of one of these tests.