I think I would rate it as a 3.5 of 5 right now. It looks great and combat plays really fluid, and your character looks great! There are a number of issues underpinning it that together make me not as invested as I was with Diablo 2 and Diablo 3. I'm not super hungry for spare time to play Diablo 4 but I think it's "fine".
However, I think Blizzard will have some nice updates to come because my problems with the game is echoed by so many others, like itemization, skill and build variety and high-level end game progression. In these regards, I wouldn't be surprised if we're looking at a vastly updated game already by autumn.
So that's why I'm happy that the core of Diablo 4 looks and feels good because that'd be harder to fix than the spreadsheet-like data.
Yes, I know Diablo 3 got a TON of complaints about the art direction but honestly my problem with it was always more about the World of Warcraft-style blocky architecture etc. rather than the colors. I always liked rather colorful games since they really make each area feel different and new, which is great when you're playing in Adventure Mode and like to switch things up.