in the book Blood, Sweat, and Pixels, the chapter on the disaster of Diablo 3's launch Diablo IV Gold, tells the story of how one Blizzard Developer played Diablo 3 for hundreds of hours, before they came across a single rare loot. When that light in orange finally appeared from a random enemy, the developer walked up to the item only to find that his class could not even use it. The loot system was so inherently broken that the pressure that comes from grinding all day, after which relief of having something of value, was not possible.
This was eventually fixed to which you could only find certain tiers of loot that would work for your class, and that the rate at which early-game legendary items were dropped raised. Therefore, even if the legendary items that you could get didn't break the game, you still felt you'd occasionally receive a slight dose of dopamine in order to keep your hands on the rope.
When Diablo 4 gets that right and implements a similar game mechanic to Loot2.0 from Diablo 3, then we're already worried about just the amount of time we'll get to sink into the game. Diablo 3's debacle is the most beneficial thing that could have happened to the franchise on a go-forward basis, and combine it and Immortal's Immortal controversy, it appears like Blizzard is able to provide a straightforward strategy for avoiding potholes should it want to remain among the best of it's most loyal players.
The community is very open about the things they don't like and has been all through the lifespan of Diablo 3, so we're hoping Blizzard is taking these concerns into consideration for those lucky enough to play large portions of the game pre-release. Although we'd like to bet Blizzard isn't too thrilled with the stream of leaked footage which appears to come after the private test.
One aspect that is part of Diablo 3 that is confirmed to return is the season-long Seasons cheap Diablo 4 Gold. These are essentially post-post-game pieces of content that refresh the ways legendary items function and also remix the game's content of the game base which adds replayability to a game that's already incredibly replayable. Seasons were available for quite a long period times in Diablo 3, so we're hoping they'll also feature heavily in Diablo 4.