Well I suppose (and hope!) they reworked the battle.net servers for this remaster, because the old servers were (and still are!) very laggy. It is very common to not hit a monster even if your fireball touches it on your screen or on the exact opposite see a monster die from many yards away of the bolt. It was also very common to to have to hit a monster several times before the server actually register the hit (and no, it’s not because of the “miss” mechanism).
On top of that, they implemented some new features that will change the data structure of the characters and even the account (bigger stash, shared stash). This wasn’t part of the original that was tested for 20 years, and I suppose it would be worth checking that everything works smoothly before launch.