To those saying it’s barely noticeable, I can only guess that you’ve been playing at quiet times when the vast majority have logged off and gone to bed or whatever! Once you complete the tutorial and arrive in Kyovashad it’s almost impossible not to see other players! Last weekend I went around Kyovashad and counted each player that I saw simply because I was curious as to just how many other players I could see at any single time, but I stopped counting once I got to 23 different players because I realised that this is much more like an MMO than we have been led to believe!
Sure, once you leave a town/hub area and go out into the world you don’t see quite as many other players, but you can certainly still see random players running around doing their own thing! A few times I would see an event show up on the map, head towards it only to get an “Event succeeded” message just as I arrived. That’s not fun at all, that’s certainly not exciting gameplay, it’s just annoying and frustrating, which, if it happens too often, makes me want to stop playing! At one point I even got a quest completed without actually doing anything for the quest other than getting close to the area the quest objective was in!
Now, we do know that Blizzard have dialled up a few things especially for the beta weekends, such as higher/earlier drop rates for legendaries, so I don’t know if the extent of seeing other players is also higher than it will be when the game goes live. I really hope that’s the case because, quite frankly, I just don’t want to see all these other players in MY Diablo experience! No, it’s got nothing to do with "social awkwardness " or anything else like that, I play plenty of MMO’s (including WoW) so I have zero issues or problems with seeing and/or interacting with other players who are complete strangers to me!
Diablo, Diablo II and Diablo III are all single-player games with co-op options. The only part of Diablo IV that is single-player is the initial “tutorial” and introduction. Once you arrive in Kyovashad that’s the end of any and all single-player experiences and from then on it’s all multi-player, bordering on MMO-lite! As others have said, Diablo IV seems to be more like a continuation of Diablo Immortal, minus all the insane, greedy monetization that Blizzard shoved into that abomination!
I’ve seen some players saying that D4 is nothing like an MMO! 
Really? You’ve got World Bosses that require multiple players to kill just like WoW and other MMO’s have. You’ve got Strongholds that require groups of players to conquer, just like Dungeons in MMO’s. You have to log into the game and a server, just like an MMO. You have to wait for respawns, just like an MMO. You see other players everywhere, just like an MMO. You see LOTS of other players with dumb, immature, idiotic names just like an MMO. Still want to try and claim that D4 isn’t an MMO? It seems to be a whole lot closer to being an MMO than it is to being a single-player game!
The real question is whether Blizzard will ever see any of these comments and, if they do, will they actually do anything about it? While they may indeed see these comments I very much doubt that they’ll do anything about it. I’m afraid it may simply be a case of either accept it for what it is or, if it’s too much for you then it’s likely better to ask for a refund sooner rather than later! 
Had problems getting in-game so far this weekend so I’ve yet to have my memories of the game fully refreshed! 
I am having very mixed feelings atm