I never really considered the "tone" of Diablo 3 a problem.
The biggest problem with Diablo 3 is they never figured out the loot grind. When the game first game out the loot grind was way too punishing to find loot you could use yourself so you'd need to use the auction house to outfit your character which really took killed the desire to grind for loot.
Then with the expansion and development of rifts, greater rifts etc and revamping of set items they swung WAY too far the other way.
Builds became entirely dependent on full set bonuses and legendary/set items became increasingly easy to find where it got to the point that you could fully gear out your level 70 character (which took maybe 90 minutes to get if you were going slow) in a day or two. Then the only goal became finding slightly better versions of the set items you already had and trying to grind out paragon levels
There was something much more rewarding about the loot grind in D2. Yes, there was still the absolute elite items that you'd need if you wanted to have a truly elite character, but there were always alternatives along the way to fully gearing out a character and there was frequently variations in your character builds if you wanted to be a little different.
Not that D2 was perfect. The introduction of the really over powered runewords along with never truly fixing botting and duping of items really changed the landscape of the game.
I'm concerned that the only thing Blizzard heard with D3 is that the game wasn't dark enough. So we're just going to end up with a re-skinned D3 that's a bit grittier in art style.