So, given several requests for information on leveling DKs, I'll accelerate my plans to blog this topic and get started.
First, my qualifications to give suggestions on leveling DKs:
1. I leveled a DK 55-80 on the beta, which is when I decided DKs were for me. I tried all three main talent trees as I went.
2. I leveled my current main, Hinenuitepo, 55-80 in less than 5 days after LK release and was the first alliance DK (second overall) to hit 80 on my server. I mostly leveled blood, and leveled side-by-side for much of that stretch with a guildie who went frost 55-80.
3. I'm leveling another DK right now, Anunnakis - who is currently level 77. This DK I started because I recently moved and became friends with my neighbor across the street. My new buddy is very very new to wow and I wanted to play some with him and help him figure out the game - he was already 55 when I joined him, so starting a DK was easiest to keep pace.
4. I've also leveled a shaman and rogue to 80, and have two other 70+ toons, so I'm familiar with leveling as other classes as well. As a side note - I'm still enjoying the leveling process and seeing Northrend again through 'fresh' level 58 eyes was a trip!
Ok, so on to it. Today I'll just blog 55-60, but in the next few days I'll cover the rest of the journey. This is not a quest guide, so if you want to know sequences of questing or how-tos, Wowhead and many other sites have more infomation on these.
When you first pop that level 55 baby DK, you won't have any talents at all. Many of the quests you'll complete in the starting area will award you talent points - so don't forget to spend them!
1. Talent spec 55-60. First, you can level using any spec, and DKs are so overpowered for leveling, mobs will fall down. But, to optimize your journey, I do have some strong recommendations. You could level unholy, or even frost, but I don't recommend it. The main reason is downtime. Even though unholy will do superb dps and aoe like crazy, you don't regenerate health like a blood dk, which will result in downtime. So, for the rest of my leveling blogging, I'll recommend blood. It doesn't matter the order you pick up talents during the starting area, but I recommend when you ding 58 and are ready to move out into the world, you end up 32/0/17. Why so many points in unholy? One word: On a Pale Horse. :) I always recommend travel speed buffs because traveling from place to place is such a huge part of the time it takes to level. After that, you'll keep adding points to fill out blood to hysteria, which gives you a nice dps cooldown when you reach hellfire peninsula and start running 5-man instances for gear and the really nice quest xp. Vendetta is the best talent in the blood tree for now, because it keeps you topped off most of the time, and when aoeing 5-6 mobs you'll keep getting health back when death strike alone isn't enough.
2. DPS as blood. You'll start out with the basic attacks of Icy Touch, Plague Strike, and Blood Strike. You'll also have Death Strike very early. It is very simple to kill mobs early on. Just:
Icy Touch > Plague Strike > Blood Strike and Death Coil when you have enough rp.
Hereafter, I'll write it as:
IT > PS > BS > DC (for those that nerd speak is anathema, sorry) :)
Try this on the dummy, or at least you'll get to practice it in one of your very first quests as you start.
If fighting multiple mobs you can Pestilence to spread diseases, but this isn't necessary. Starting area mobs die quickly and easily, but it will get you used to which buttons to hit. Early on until you're comfortable, try to pull one mob at a time. By the time you're 58 you should be comfortable enough to do some aoe along with the standard rotation to bring down multiple mobs quickly.
3. Miscellaneous: If you're running your DK as a well-funded alt, there are many things you can do to 'buff' your experience, but none are necessary. You can buy flasks/elixirs, you can buy boe gear and enchant it. You can have buff food waiting for your DK. You can buy some quest drops for turn in instead of having to farm them. You can buy the cold weather flying book, flying skill, mounts, and dual spec. And on and on. I did all this for my DK main on my race to 80. But, if you're not rich, or don't have a 'sponsor' (rich main on the server), it's still not a problem. For Annunakis, he is my only toon on his server, and I've still been able to buy cheap gems for slots, buy my flying skill and mount, and purchase dual spec and cold weather flying when I dinged 77. Quest rewards and drops should keep you in enough gold for all this, even if you're not an AH king.
I still recommend primarily leveling through questing; it's just so efficient. It can be rewarding, however, to run some 5-mans, especially the first time for quest rewards/xp, and yes, battleground leveling is still viable as well.
Oh, and go to Outlands immediately at 58, just like you should head for Northred at 68. The xp from killing mobs AND quest turn in is much better than staying in the 'old' zones.