I think the problem is just that the co-op experience is in it's infancy. I look forward to a co-op horror experience designed to emphasize terror isolation and suspense.
- design it so that there are times when your co-op buddy will appear as monster (maybe after being poisoned), so you'll have to shout over the headphones "don't shoot me" "which one are you" "i'm at your 6. no i mean your 3. ahh!"
- design it so that your co-op buddy is mostly separated from you. mostly joining up when there are no baddies, to offer a break from the tension. and maybe swap items.
- design it so that audio cuts out at key points. like one player can see the other, but suddenly his mic goes on the fritz - making it hard to warn them.
(think of that scene in Alien. where one person has to crawl around in the ventilation tunnels, while another person tries to explain where the monster is, and which way they should run).
- design it so that it seems clear to player 1 what player2 should do ("go pull that switch, p2! what is taking so long!"), but design player2's path so that it's ridiculously hard to do the obvious (tons of critters around the switch that p1 can't see). foster tension between the players.
- record what one player says (perhaps based on volume or intensity) and then play it back later when, they are being attacked again. sow confusion.
- and most of all, play with the illusion of shared reality. ("do you see this thing on the table?" "no man, i don't even see a table. I see your dead body though." "uh, what?") DeadSpace seems like a perfect game for this sort of insanity effect. in fact, i thought it WAS in there. i was going to buy it BECAUSE i heard there were purposeful differences between each player's perceptions in co-op. uh. hmmf.
anywho. just because the lights go out, doesn't mean people shut up in a theatre. the movie has to be designed to demand their attention and respect. i think co-op is in a similar boat.