I honestly agree with the OP, Diablo 4 just kinda feels "fine", but definitely not exciting. I think they're really missing the part where in both D2 and D3, the player characters are AWESOME and BADASS.
In fact, it really sticks out to me that the player models are also used for a lot of enemies and NPC's. It kind of lends to the feeling that "you're no one special". Which, if that's what they want to do, that's fine. But I think that having these cool, heroic classes that are all "in their prime" is very much part of Diablo's success, and it feels sorely absent in D4.