Your highest DPS was 7k? You should have people (Warlocks, Mages, Shadow Priests mostly) pulling ~10,000 DPS. That's kind of a moot point, though, since it's largely a result of 4.0 changes and you could definitely do it without that much DPS.
If you can't get people to stun and snare the Valkyrs, you're screwed. If you can't get people to move away from Defile, you're screwed. I mean, that's basically the whole fight.
Phase 1: Offtank manages the adds and people who get the disease run behind those mobs and get cleansed so the disease hops to the adds and kills them off.
Phase 1.5: Tanks manage the Raging Spirits, healers and DPS spread out between the two tanks and kill the Raging Spirits and the Ice Spheres before those get too close. Earthquake time, run in.
Phase 2: Tank LK in the center, facing away from the raid. Everyone groups up behind him until the Valkyr (Valkyrs in 25) comes, then spread out to reduce issues with Defile. When he picks a Defile target, that guy needs to go either north or south and deposit the Defile away from the path of the Valkyrs. Uh oh here comes Remorseless Winter again, get to the edge (I've always used the eastern edge).
Phase 2.5 same as Phase 1.5
Phase 3: No more Valkyrs, look out for Defiles. When he does Soul Harvest heal the crap out of that person, they get sucked into Frostmourne and have to either heal the NPC inside or help him kill the bad guys.
Oh **** guys he just wiped us!
Phase 4: Free Purples, enjoy.
It's a great fight, really. Mostly it's about coordination, which is honestly the hardest part of any raid. Either people pay attention and learn from their mistakes, or they do not.