There's actually not a lot I can say about the single-player story in Dead Space 3. That is, it doesn't do anything exceptional in terms of presentation or content. Playing it in co-op, however... well, that's a different issue. I've recently completed it with a friend and that experience was undoubtedly unique. The reason I found it so interesting is because, unlike other co-op games I've experienced, DS3 attempts to provide a story that changes based on the way it is experienced.
The story (if there is one) in most other co-op games I'm familiar with usually revolves around one character, and is experienced individually by each player. In Diablo 3, for instance, the main quest is pretty much the same for all players but each player sees story clips that are unique to the character he's playing. There is never any acknowledgement of the fact that you're a group of people instead of just one. The same approach can be seen in other co-op games. One exception I can currently think of is Portal 2. That game has a co-op mode whose story revolves around both its characters. Besides them not being very talkative, though, that game's co-op campaign is separate from the main storyline. In that sense it's more along the lines of conventional multiplayer, I think. Resident Evil 5 also manages a a story with two main characters, but there the co-op character turns into an AI partner when the game is played alone.
Dead Space 3 goes in another direction; it wants to provide a proper single player experience that will adjust seamlessly when experienced in co-op. Thus, when played alone, Dead Space 3's single player story focuses entirely on Isaac Clarke, the series' resident protagonist. In this mode, John Carver is just another supporting character, albeit a more central one. When played in co-op, however, all the game's cinematic moments are adjusted to have Carver in on the action alongside Clarke. I like this approach because it doesn't sacrifice the single player experience for the sake of co-op. The developers intended to provide a decent story that would work when played alone or with a partner, without
sticking a crappy AI who'd get itself killed most of the time (I'm looking at you, RE5!).
The problem is that even though it creates a seamless experience when playing as Clarke, the same can't be said for anyone playing as Carver (which I did). Even though the game's story moments are tweaked to accommodate two players, these moments are entirely focused on the character of Clarke. And I don't mean that only in the narrative sense. Literally, when an event happens, the camera shifts to Clarke, leaving Carver somewhere out of sight. To be fair, there are Carver-focused missions in the game that are
exclusive to the co-op mode. However, they're not part of the main
storyline; they can be skipped or even just missed while playing. The real reason this is an issue is because it highlights the fact that Carver isn't really relevant to the central plot. And it's true, in a way. Playing as Carver in co-op produces a sensation of being almost entirely disengaged from the story, and worse - like the story is intentionally leaving you out. It makes it clear that Carver's inclusion in the game's story to begin with is for co-op purposes only.
My guess would be that the story making sense in co-op came second to providing good gameplay experience. As a fan of stories I'm a bit disappointed, since that attitude effectively ruined my experience of the story in co-op (even though the gameplay was fantastic). I still like the idea, though, and I think that their approach could work if they adjust the story appropriately next time (assuming there is one). All I know is I would love to play a co-op game that embraces its two protagonists fully.