I think some people just miss the inertia and immersion of the old, corridor-based, narrative-driven, cinematic-laden Metal Gear Solids. They provided a special kind of fix that no other franchise could emulate. This entry’s more open, less focused on plot, and offers an experience that’s a bit less unique, more like other shooters and action games. But I think after so many traditional Metal Gear Solids, we were due for something totally different, which opens up previous games’ gameplay while incorporating stuff from other franchises and competitors. Also, it still feels undeniably like Metal Gear Solid. I’m still really, really digging it.
As for the open world design, it’s only open-world in comparison to previous Metal Gear Solids. I mean, it’s otherwise fairly focused, even in terms of the main narrative, and I never found myself without purpose or direction.
I would compare much of it to STALKER, in fact, which I also found balanced open world design, and a truck load of missions and side quests, with a sense of progression and a clear goal.