The book Blood, Sweat, and Pixels in the book Blood, Smudge, and Pixels, chapter 4 on the disaster of Diablo 3's debut tells the story of how one Blizzard producer played through the game literally hundreds of hours, before they discovered a piece of legendary loot. The moment that the orange light emerged from an opponent Diablo IV Gold, he went to the item only to find that his character class couldn't even use it. The system of loot was so fundamentally flawed that the excitement being a slave to the system for many hours after which relief of actually getting something special, was destroyed.
The issue was later fixed to where you could only ever discover certain levels of loot that would work for your particular class, and the speed at which famous items would be dropped raised. Therefore, even if the legendary items you got were not a problem however, you would sometimes get a little bit of dopamine, which kept you hooked.
If Diablo 4 gets that right and uses a similar loot game mechanic to Loot2.0 in Diablo 3, then we're already concerned about how long we'll need to spend playing the game. Diablo 3's debacle is the most beneficial thing that could have happened to the series on a continual basis. When you combine this and Immortal's Immortal controversy, it's like Blizzard has a simple roadmap of potholes to avoid should it want to remain on the right side of it's the most faithful players.
The Diablo community is open about the things they don't like and has been doing so throughout the development of Diablo 3, so we're hoping Blizzard is taking it into consideration from players fortunate enough to experience large portions of the game pre-release. Although we'd bet Blizzard isn't a fan of the massive leaks of footage that seems to follow in the wake of one of these private tests.
One element in Diablo 3 that is confirmed to return are the time-limited Seasons. These are basically post-game items of content, which change the way legendary items work, and remix the content of the base game giving replayability to the game that's already extremely replayable. Seasons were supported for a considerable amount times in Diablo 3 buy Diablo 4 Gold, so we're hoping they'll also feature heavily in Diablo 4.