I actually think the way they are implementing Co-op could potentially add a whole new level of scary aspects. Issac's co-op partner Carver is in the beginning stages of dementia. As you and a buddy are playing, you two aren't necessarily seeing the same things. Imagine, you are both walking down a dark cramped bloody hallway with weapons drawn. There is no sound, but all of the sudden your buddy starts letting loose with all the ammo he has, firing around you, but you as Isaac see nothing.
If I have to walk around with an AI Co-Op player because some 13 year old wanted to play with his friends I'm going to have a very bad time. If you want to play Dead Space with friends, you really don't want to play dead space. You want to play Halo... Go play Halo.
That's because the demo seemed to highlight a couple of things about the storyline and the early mechanics of weapon testing/creation, not so much the super jump scaring that I remember in the first two games. I'm thinking the Co-Op will bring a new light to the horror that is Dead Space, but then again, I might be alone in my thinking.
I think a lot of people (the anti people) are making a lot of assumptions that have nothing at all to do with facts. Prime example: if you've researched DS3 at all since co-op was announced, you'd know that if you play single player, you see Carver only very rarely.