Sorry, I'm at work, so I don't have time to read all the responses.
Sorry for the length of this post; I didn't realize it would be so long until I was done.
I started unholy on my first DK and loved it so much I did it again on my next few. I've never had any problem whatsoever handling elites 4+ my level and packs of up to 10 regulars 2+ my level. This was, of course, before the DnD change and the Glyph of Death Strike nerf, but I'm confident you should still be able to handle about the same.
Coming from a mage background, I am a major AoE whore. If there's an opportunity to AoE, or a viable way to quickly create that opportunity, I always do it. If I only need one more kill for a quest, but there are five of that mob within pulling range, I pull all five. I almost feel like I'm wasting keystrokes if I don't have at least three or more mobs mad at me at all times.
I tanked everything regular in Outland except SH, SL and SV (just went to Northrend before I found any groups for those, or I would have) as unholy with only quest gear and a mediocre healer, and duo'd all of Auchindoun except for SL with a good priest at level 67.
Basically, for unholy DPS, go with the biggest, slowest, hardest hitting two-hander you can find for your level because Scource Strike and other attacks dependent on your weapon damage will be your primary attacks. Keep DnD and Unholy Blight up as often as possible.
I recommend aiming for this spec, starting with unholy unless you feel a need to pick up Butchery early to help keep Unholy Blight and Gargoyle up. I really have only used the gargoyle on bosses and elites, and even with the nerf he's definitely worth a point.
Here's the rotation I've always used, and I'm typically doing around 40-50% of a group's DPS on my own:
Make sure Bone Shield and Horn of Winter (now that it doesn't cost RP, but gives you 10 now) are up at all times.
Then, just spam your strikes. Whenever Scourge is up, use it, whenever you have a blood rune do a Blood Boil. Use a DoT timer and if you think the group will last another 10 seconds or so, renew with PS-IT when they're under 3 seconds, then re-Pestilence.
- Grip (if needed, or to help pull casters to the tank)
- Death and Decay (be sure to aim it so that most mobs hit it almost immediately, I usually aim with myself at the back edge and pull a caster to me to stack diseases before the rest get to me)
- Plague Strike
- Icy Touch (yes, PS first, then IT unless you're not gripping, then IT while you're running to melee range)
- Pestilence (once everything's in range, use the glyph for an extra 5 yards)
- Unholy Blight and Blood Tap (Don't wait for the GCD, hit it right after Blight)
- Blood Boil (or another Pestilence now that it's not on CD anymore)
- Whenever you have no rune abilities to available, burn your runic power with Death Coil
- Pop Icebound Fortitude as soon as you can see you're in trouble, and Anti-Magic Shell if there's a caster out of AoE range that's putting a hurt on you and you can't grip him yet
Minor glyphs: Pestilence, Raise Dead and Horn of Winter, in that order as you level.
Major glyphs: Death and Decay and Ghoul (non-negotiable), then choose between Death Grip (only for chain-pulling while soloing, useless otherwise), Bone Shield, or Rune Strike.
Keep your ghoul up as much as possible. Before the glyph of ghoul, I averaged about one stack of dust per level or two, so it's really not hard to keep him alive, even when using him to round up 5-10 mobs for you. Also, always be aware of your surroundings and help the healer out by gripping or Death Coiling mobs bee-lining for them when needed.
At 80, a lot of this changes, but while leveling, this has been extremely successful for me. I've literally only seen 4 DK's who were within 4 levels above me who ever did more total damage in an instance, and no other class that was within that same 4 levels has ever done more total damage in an instance. I typically do 40-50% of a group's DPS in an instance doing all this, and usually only about one pull in an instance do I not have the highest damage for that pull, and it's always because I messed up somehow.
Good luck! Enjoy!