Valithria is the already famous/infamous 'healing fight' of ICC - and what a fun one it is! I love a fight that switches things up and gives healers a chance to shine. As a DK, however, this fight is a fairly mundane one (conceptually) of killing repeated waves of adds to keep them from stopping your healers. Still, your dps is very important to successfully completing the encounter, so let's get to it!
Pull/Phase One: I DON'T use my Army on the pull in this fight. In fact, I usually save it for the last 5% of the fight when adds are going crazy and threatening to wipe the raid - then it's ok for the Army to provide the last few seconds of add-tanking and may provide a few seconds of healer breathing room, since dps is not the main point anyway. Control is. Use your ghoul/gargoyle on cooldown just to help make sure you're controlling adds.
Kill Order: As you should know, the key to succeeding in the fight is killing the different types of adds in a predetermined order and manner. Depending on the fight, they kill order should probably be: 1) Blazing Skeleton, 2) Suppressors, 3) rot worms, 4) Risen Archmage, 5) Abomination, 6) Zombie.
Blazing Skeleton: IF you have a ghoul (unholy) be very careful on this mob. DON'T hit it with an Icy Touch or Death Coil from range if it will sic your ghoul on it. Also, stay away from the skelly until it's moved into melee range of it's target (use target of target). Ranged should be nuking it. When ready, unload on it. Why? Because it doesn't start its Lay Waste ability until it's in melee range. If your ghoul attacks it, or you start meleeing it early, it will get a couple more ticks of lay waste, and your raid will wipe.
Suppressors: on 10-man, I usually DON'T hit suppressors at all. This is usually the ranged responsibility. This is because we have lots of ranged, and they're good. We have the tanks pick up all the other adds and drag them to in front of Valithria's head, minimizing the need to run around. DPS time increases overall. Suppressors are the #2 priority, however, and if your raid has problems getting them down fast, then you as a melee should still run to them as soon as they spawn. IF you must kill suppressors, at least ask your raid lead to assign you to one side only.
Worms: when your aboms are dying, the abom tank calls for aoe on him. All I do is drop a death and decay on the tank - I actually stay away. Two reasons; all the aoe kills the worms fast and secondly, if you pull agro on worms, it's a recipe for a quick death.
Archmage: needs to die quickly if other things are accounted for. The dps it does isn't bad, but the slow effect can be killer. These guys in particular seem to like to go straight to the raid healer, and with the tanks picking up so many adds, I often find myself grabbing a mage and tanking it for a bit until it can be taunted by a tank or killed. Its magic damage can be easily AMS's through, and silenced when it casts.
Aboms: when I have a chance, I dps the aboms. More often than not, I'm running around putting my diseases up on all the mobs to spread ebon plaguebringer so the casters will have better dps. That's being a team player, and doing your job well.
Zombies: Ours die fast, so I generally don't get close except to spread diseases. Their explosion when they die can be bad, and even if you don't die, it puts strain on your healers.
To sum up: DPS the adds in the correct order as quickly as possible. Stay away from things that will kill you. Spread diseases. My dps on this fight is poor compared to others, but if most of the adds are tanked closely enough, it's not too bad.
After that, it's on to the final two bosses of Icecrown!