It seems at the current state of the game, well, not really thought through or designed for a hardcore experience. Allow to share my personal observations and remarks:
1.) Some mechanics are rather hard to handle as they seem random and unannounced. E.g., Oscar Reede’s explosion upon death was hard to see and killed several
2.) Generally boss mechanics are far from self-explanatory and sometimes unpredictable. Especially if you are outside of the view of the boss you can be hit (one shotted) by a something you were unable to anticipate.
3.) The world bosses seem rather imbalanced for hardcore, thus I was always alone or a maximum of two or three players there (no matter on normal or veteran setting). As a swipe hit instantly kills a character and only Wizard has a cheat death mechanic it seems rather impossible to vanquish one of these without taking advantage of some behavioural traits (terrain).
4.) Generally it seems that melees are at a disadvantage at the current state. Pressure plate events only work if you stand still and allow yourself to be swarmed.
5.) How is PvP going to work: are you forever dead? Or will a PvP debuff be applied that lets you revive?
É sicuramente prematuro parlare di HC ora non conoscendo a fondo il gioco. Già si crepa sul 3… 
Si potrà cmq provare con un nuovo pg.
PVP in Hardcore
Hardcore players can partake in [PvP]
If a Hardcore player dies in [PvP] the [death won’t count as permadeath and the player can continue playing the storyline.
Currently, permadeath is not a thing, but they’re looking for feedback. If players want that, they’ll implement that and have hardcore permadeath be a thing.
I played a rogue to 25 on softcore and a sorcerer on hardcore in the beta and I completed all the dungeons I could find (renown showed one side dungeon not completed for me) in hardcore on veteran difficulty. I killed the world boss with my rogue. However when I had the sorc ready the world boss was not available anymore (was hoping I could finish the beta there and go out in a blaze of glory).
Overall I’ll say that if things stay the way they are then most hardcore deaths will either be due to random boss spawns like the butcher or the world boss. The butcher is just crazy difficult for some classes. The first time I encountered him was on my rogue and he killed me very quickly without much chance to actually fight back.
I managed to defeat the butcher twice on my sorc though in hardore (much stronger character overall) but in both cases it was a close fight and I thought that he might kill my character. The butcher is just so crazy fast and a very tough opponent when you are in the middle of a dungeon with other enemies around and this is the kind of enemy that would push people away from hardcore.
Also the world bosses will likely cause a lot of deaths simply because of instant kill abilities that can easily hit you if you don’t have strong movement abilities to dodge them. Also when you are in a world boss fight and someone actively wants to sabotage you to kill you the world boss would be an ideal place as there are 7 other players around which means 7 possible sources of problems. And right now nothing stopped people from moving into a world boss fight at level 1 for example. They might lose their character but you could lose your level 50 or 60 character as well. That would seriously suck.
This is the problem with open online worlds. You have a lot of potential sources of griefing where people just want to mess with you and that is especially exciting for them if the result could be you having to start again from the beginning.
I’ve played Diablo 3 on hardcore basically since the first season in RoS but I’m not sure I would be willing to actually do the same in D4. In D3 I can play on my own and outside of disconnects that could kill my character it is all in my hands. I’ve lost characters because I made stupid decisions but those were my decisions and not someone else pulling some weird stunt…