Diablo 4 has all the bad mechanics of Diablo 3, with the atmosphere of Diablo 1.
The art department absolutely nailed it in Diablo 4. The Diablo franchise had a trend of moving away from their super atmospheric feel of Diablo 1 into more of a 'cartoony' looking franchise. It is nice they went back to Diablo 1 in this regard, but this is the only thing positive about the game.
The game mechanics need to be ripped out and redone. There is essentially ZERO build diversity in the game, especially compared to Diablo 2. I have no idea why, but Blizzard took out mana/resource management game mechanics in D3 and carried it over to D4. No mana pool scaling with level, investment in leech, or most other forms of mana regen. This is to the point where there are only 1, maaaaybe 2 builds for each class, and these 'meta' builds are simply meta because the voice acting of the character isn't constantly complaining "Low on mana!"
Further, unique items only drop for your class, and cannot be traded with other players, so there is no organic transitioning into new characters. Even more: you can't really give your other characters new gear since gear drops at your character level regardless of the item tier on the level. If you are level 86, and get a 'sacred' item which is good for World Tier 3, you cannot use it on another character until they are also level 86 (and in world tier 4). This item will not have higher stats because of the level equip level. In fact the exact item could drop for a level 50 character, or even drop with better stats, but be equipable at level 50 instead.
The game also lacks the content of Diablo 2. Diablo 2 was about grinding your way through Hell difficulty, farming each act until you could progress to the next. Diablo 4 goes straight to generic dungeon running until max level and max gear - the same dungeons over and over again, just with slight scaling.
And finally, the level scaling mechanic needs to be stripped out. Most of the damage dealt/taken from mobs is purely on your level. I got carried into World Tier 4 at 52, which was too early, but without much for gear upgrades, we were trivializing the base level 73 of the World Tier 4 content at level 63/64 simply because we overcame the worst of the level scaling. At level 64 and above, we were trivializing tier 4 more so than World Tier 2 or 3 when we were the appropriate levels for those.
My current content is farming gold so I can afford to reroll the gear that I have that have 3/4 of the best in slot rolls for my class. Rather than running 3-4 dungeons per roll, I am going to go play other games that have content until I get enough money in Diablo 4 to afford some more rerolls lol (which of course will be never).