There's potential. In it's current iteration there's just not enough to keep you hooked let alone being overly interested in doing more than 1-2 characters fairly deep (lvl 70+).
My first play through was enjoyable. I stuck to the main questline, progressing through with a friend. It was a good story, good voice acting and the gameplay was fun with my Necro. An occasional issue that is expected in a new game such as lag spikes, stuck characters, a bugged quest; nothing game breaking. Once we finished the story we started unlocking all we could with paragon, helltides, renown, altars. Our characters became very strong, dispatching world bosses in no time, and it felt good.
During my downtime while not with friends I'd experience some of the other classes and the current builds available to them. All very enjoyable and once my first character finished the story things on the alts were a breeze. Reaching level 50+ takes no time at all once everything has been unlocked and your characters feels much stronger much earlier.
This is where the problems reared their ugly heads, inevitable for such a game in its first year. Beyond your first character getting to the 70+ point with alts at 50+ things are just stale and repetitive. Yes, ARPGs have lots of repetitive aspects but when that activity is stale...it just doesnt feel good. Diablo 4's endgame is just lacking and the first Season wasn't enough to build onto what was there. We still go do the same 5 things with slightly different environments.
The endgame just needs lots of work. No reason for me to currently go back and touch any of my characters. People will use POE as an example of things to do in endgame...well that game has 10 years with 4 leagues per year, it takes time.