Ever since we first heard about the spell, I have been without a doubt drooling over Army of the Dead. This level 80 Death Knight spell allows you to summon an army to your beck and call. In specific, it's a 6 second channel spell that summons several weak undead to taunt and fight your enemies. In addition, you take less damage equal to your dodge plus parry chance. It costs one of each basic rune type, and has a
10 minute 20 minute cooldown (You see how swiftly things change in the Beta?).
You can see the spell in action above (video found on Deathknight.info). You may notice some problems. For example, some the ghouls stand around and do nothing. In addition, the damage reduction on channeling does not seem to be working. Still, it's pretty cool seeing all those ghouls decend on the enemy, even if they are lowbie scarlets.
The question, though, remains: What will we use this spell for when all is said and done and it works well? For now, it looks like it's going to be mostly a panic button. As a tank, that six second channeling session is problematic because it's six seconds in which your DPS can outpace you on threat. If your DPS is already running or can be coerced into dialing back, however, that reduced damage could be nice for weathering a hairy spot.
DPS can probably use it a bit more freely for emergencies. If the tank looks harried, see if some taunting ghouls can help out. Since they stick around after the channeling for a bit, they may even bolster your personal DPS, depending on how much they do.
Army of the Dead overall, though, is probably going to need to find a niche. It's an awesome high-concept spell, but it may still need some work on paper, or else it will end up a novelty spell like a Warlock's inferno, primilary trotted out for novelty or to wreak havoc in mass PvP. A start would probably be to remove the channeling (so it doesn't interfere with the Death Knight's tanking or DPS duties) and upping the HP on the ghouls so they aren't so easily and immediately taken out by AE attacks.
It's still a very cool spell regardless, and I hope to see it find some use once it gets debugged and tweaked a bit