This was based on my actual play experience during the beta weekend. It was not just world bosses, or even Helltide events; I would have random players contribute to pack clearing in the open world like it was Guild Wars 2. That's not really the experience I'm looking for in an aRPG, I play GW2 for that. If your experience in the live game differed, that's cool, but I found it obnoxious enough that, combined with the level scaling, I was put off the game. And that was probably for the best given what they did (or rather failed to do) with Lilith.
I think that was largely an artifact of the beta weekend. Either that or they turned a dial like
@Whizbang Dustyboots suggests.
Unless it's a Helltide (or similar) or a World Boss, I pretty much never see another player. In fact, I would say I often literally go for several hours in the open world and never see other players outside of towns, Helltides (and similar - there are often season events like that).
That's not to say it was a good decision. I don't think it was. I think the open world has two huge problems:
1) It's incredibly bland and visually boring and undifferentiated. Like, sure, there are five or six biomes, maybe slightly more if you're generous. But every single location is rendered in such a way as to be completely forgettable. I've got hundreds of hours logged on D4 and if you showed me a random screenshot (with no map/minimap) of an area, I could probably tell the zone, but very little more.
Whereas with say, WoW, even early on, unless it was somewhere I'd never been, I could have reliably "geoguessed" pretty much every location fairly precisely. And WoW's map is much larger.
Also the combination of bland and "always the same" means you never have a reason to actually look at the world outside of Helltides (and similar). You just blast past it on horseback looking at your minimap and trying to keep your speed as high as possible.
Absolute and total fail/miss on Blizzard's part.
2) It adds nothing to the game, gameplay-wise apart from tedious collectathons.
Again, big miss. D4 wants to you spend either countless hours trying to collect boring statues and waypoints "legitimately" or to go online and follow some maps. And you need to do all the waypoints and fortresses every. Single. Season. Which is just boring. Really boring. They made it so the fortresses give big XP to try and compensate but it's so fundamentally dull. It's not as time-consuming as say, going through PoE1/2's story, but it's somehow vastly less engaging.
And nothing else in the world is worth doing, really. Especially the countless quests - some of them are quite well-written, but they're all boring and featureless gameplay-wise and totally unrewarding in a game that is very much about efficiency and doing the right thing.
Most of this is because they made it a FIXED open world.
They did that because they made it a SHARED open world.
To share it as much as they wanted, it has to be fixed.
And the precision difficulty control, which is kind of necessary for the setup they've created means there's no texture to the world either. There are no hard or easy or farming areas, really.
So I think that + fixed dungeons was a VERY bad idea, and I don't think D4 will ever quite redeem itself unless they change that. But it's made a lot of money, a crazy amount of money, in largely part by selling microtransactions, so I don't know if Blizzard care.