It's a fact of life. Burning Crusade is about to become obsolete. All those rep grinds, all those exalted purples, all those dungeon keys are just so much pretty little bank space wasters. Sure, gear lasts a little longer into Wrath, but for the most part, we're still going to be leaving it all behind for the new environs of Northrend, while Outland becomes a bump in the road to 80.
For Death Knights, this is probably still true. We're going to want to rush to get through Outland so we aren't desperately trying to find groups for Utgarde Keep while everyone else is getting Kel'thuzad on farm mode. But that means we'll have only the bare minimum of faction, no grind, no turn-ins, no purples. On the whole this is probably pretty much OK, since we'll be able to get good gear right out the gate in Howling Fjord and Borean Tundra. But is there still a reason to grind some of the Burning Crusade factions, or even to pay attention to them while leveling up?
Let's look at some of the factions you'll probably come across while speeding through Outland and what benefits they can offer the leveling knight.
This is the first reputation you'll encounter as you start leveling up in Outland. It's one of the most annoying reputations to grind effectively, since you need to either hold off from doing any quests until you're honored from Hellfire Ramparts and Blood Furnace runs, or you need to do tons of Shattered Halls runs later on. Luckily with Northrend coming out, you still don't have to worry. This faction is probably infinitely skippable.
One thing I have seen a lot of Death Knights doing is grabbing a pair of Grunt's Waraxes or Footman's Longswords. If you're looking to start dual wielding, these aren't the worst you could do, although they aren't exactly the greatest either. Since they're very fast, you'll be missing out on the weapon damage from your strikes. It's best to find a slower main hand weapon to take advantage of this. Even if you're planning to tank on your way to 70, you're better off using a 2-hander to avoid getting parry-gibbed.
This one I generally get to revered on all my characters just because it's so easy. 360 Unidentified Plant Parts, and you have honored. Once you do that, you can start in on the other quests and be revered before you hit 62. This is also a nice thing to start collecting now on one of your current 70s. If you have a decent AE or reactive ability, it's very easy to simply mow down the Mudfin Frenzies in Serpent Lake. They seem to have a very high drop rate, and it should only take you around 2 hours tops to get what you need. If you're not in the farming mood, the parts have also stayed relatively low cost on the AH last i checked.
This is useful not only for a quick boost to your Death Knight's reputation, but there's a chance at getting the Uncatalogued Species. Once you've turned in all your plant parts, if you were lucky enough to get these, you can turn them in for a solid amount of experience. All in all, a nice quick boost to your Death Knight that you can start on now.
As far as the rewards, The Strength of the Untamed is a nice solid tanking necklace, which may serve you well if you want to try heading into Utgarde Keep once you hit the shores of Northrend. The Arcanum of Ferocity, if you have the spare gold, could add a nice boost to your first few Northrend helms as well. You can also always consider it a head start to getting a Hippogryph, if that's your pleasure.
This is another one that's relatively easy to start leveling up now. Arakkoa Feathers will get you reputation up to Honored with the Lower City, and an bag that contains potions, scrolls, and a piece of magical armor. The Ravenguard's Greaves of the Battle, if you're lucky enough to get them, should be a sizable upgrade to Greaves of the Slaughter, while the Ravenguard's Baldric of the Battle is comparable to The Plaguebringer's Girdle.
As far as whether gathering the Arakkoa Feathers is worth it, that's a more complicated proposition. If you're leveling your Death Knight as an enchanter, you might want to consider getting Honor for the Enchant Ring - Stats formula, though. Otherwise, the rewards just aren't really worth the time. Plus, with around 1000 Arakkoa Feathers needed to finish the turn-in, they're a lot more tedious to collect than the plant parts. You could buy them on the AH, but at 80 silver to 2 gold as the average price as long as I can recall, it'll cost you a lot more.
In short, turn in a few handfuls of feathers if you'd sort of like to try your luck at getting the Ravenguard armor or if you're leveling your enchanting while you level your Death Knight. Otherwise, don't sweat it much.
The Kurenai/The Mag'har
These guys are pretty straightforward to rep up with. You can lay into their quests right away, and then slice away at Boulderfist Ogres and turn in Obsidian Warbeads to get reputation after that. Unfortunately, there's not much they have to offer a Death Knight. They do have Talbuks at exalted, so if you're interested in trading in your Deathcharger for a strange overgrown antelope-ram hybrid thing, you might consider doing some bead collecting here.
These are another group of guys who you can start on now. Oshu'gun Crystal Fragments are usable to gain reputation up til friendly at Aeris Landing in Nagrand. It should be relatively easy to slaughter enough Virani for the approximately 120 crystals you'll need to hit friendly. Before you hit friendly, be sure you have A Head Full of Ivory in your quest log. It disappears at friendly, so take it and get some extra reputation. Once you have friendly, you can turn in Obsidian Warbeads. Or you could wait until Netherstorm, but let's face it. By the time you get to Netherstorm, you'll be in Northrend instead.
If you manage to get to honored somehow, Gift of the Ethereal is a main hand sword you might look at if you're dual wielding. Unfortunately, at 2.0 speed, it's not as slow as you'd optimally want for your mainhand weapon. Also, more reputation equals a better chance at good gems every month. Outland gems will probably drop in price pretty drastically in Wrath, but you may be able to use them for skilling up, at the least, or for socketing any socketed gear you get leveling up.
Overall, while the Consortium are one of my favorite Outland factions for sheer cool factor, they just don't measure up for the Death Knight. Sorry guys, you'll have to update your stock for Northrend!
Aldors and Scryers
The denizens of Shattrath have technically kissed and made up, lore-wise, but the rivalry and the associated quests and items still exist in game. This is actually a faction race you may want to stay out of. None of the rewards are too amazing in the face of Northrend, and it means you can walk around Shattrath freely.
Still, if you do decide to choose a side, Scryers have more going for them. Their tier is more well-stocked with vendors, and you won't die if you fall off their elevator. In addition, they'll be a lot easier to rep up with. Thanks to the Blood Elves of the Isle of Quel'danas, Sunfury Signets are a dime a dozen... Or at least, no more than a few silver each. You should be able to collect quite a few before Wrath of the Lich King goes live. If you gather enough to go honored, you'll be able to grab shoulder enchants, which can't hurt, if you don't mind tracking down the Arcane Tomes for them. Scryer DPS shoulder enchants have the nice bonus of giving critical strike rating as well, meaning they'll work for both our physical and spell-like abilities.
Is it worth it?
Honestly, I'll admit it. The biggest reason I'm planning and grabbing reputation turn-in items for my Death Knight is that I'm a reputation nut. I like seeing those little green bars go up, and if I can plan out my leveling so that I can send them up in the most efficient way possible, I do. If you'd rather not worry about reputations, you probably don't have to. Just wade through the bodies, sword swinging, until such time as you hit level 68 and hop a boat or a zeppelin headed north. If you want a mount or an achievement from Outland later, you can come back at level 80 and solo or small group it with ease.
Still, you can grab a few nice things if you're lucky, and having a reputation goal can be just the thing to get you through that long lonely 58-70 grind while you beg your friends not to level so fast. So, I'm still glad I gathered those Unidentified Plant Parts and Bog Lord Tendrils, and I'm looking forward to being the most beloved Death Knight in Zangarmarsh come expansion.