One word: OOOOhhhhhh YEAHHHH!!!
It was a ton of fun racing to 80 - as I mentioned Monday, once I stopped trying to get there first.
So here are some thoughts I had during levelling and reflecting on the process of getting to 85 and the first things to do when you get there.
1. Frost spec works like a charm. As I mentioned prior to expac release, I was going with a 2H Frost spec for levelling. It worked fine for every situation I found myself in. Generally speaking, mobs died before I'd expended all my runes and runic power, even in Twilight Highlands.
Start off with maxing Frost, then add bladed armor, and from there take your pick of talents. You get burst, mount speed, and don't have to worry about diseases. It still works very well in 5-man levelling dungeons. In fact, howling blast on trash packs is a wonderful thing, and overall the rotation was pleasing and easy to maintain! For questing with normal mobs, all you do is drop off your mount, preferably next to one mob while you death grip another to you. make sure they're both in front of you, howling blast, obliterate, frost strike if full of rp, and obliterate/FS until they're dead. There's a good chance you won't even need to use your blood runes, but if you do you can toss a few blood strikes in there.
2. Stick with an area till it's done. Technically, you can go to Deepholm when you hit 82, but if you're not done with Hyjal/Vashjir, I recommend staying until you have the loremaster achievement. Do the same thing in each zone. Several reasons for this: quests are linear, and quest rewards get better later in chains. Also, you want to get as much 'free' (non-grinding) rep for each faction as you can.
3. Find each of the 5-man instances as early as possible, and run each of them at least once. First, they're fun! Also, as you enter each of them, there are a number of quests just inside. These have nice xp and often item rewards. Finally, you'll get familiar with each of them before you come back on heroic difficulty, and it will help you a great deal to know what to expect. Many people have written that it's not efficient to level by farming 5-mans and they're right. If you really love instances, feel free, but running them over and over is a slow way to level, especially if you're not using rest.
4. Keep every meat drop you get. Eventually you'll use the meat for cooking (or if you refuse to cook, sell/donate it, but do not vendor it). Especially crocolisk tails. This meat is used for the best individual buff food for dps DKs. Side note: the best place I've found to farm this is next to the Tol Barad keep in the swamp. Any other suggestions? There are lisks up and down the banks of the river in Uldum, but they're farther apart.
5. Buy the highest speed mount you can afford. Travelling fast has always been and always will be the biggest boon to levelling. And if it feels expensive, it's really not too bad. From 80-85, if you have nice big bags to allow you to vendor greys, keep boe greens for disenchanting, and so on, you can easily make 5-10k gold! I had 7 thousand more gold at 80 than 85, and that doesn't include enchant mats or my mining. I leveled fast, and sold the first 20-30 stacks of each or I got in part to make even more gold, which I did. Even now, where there are plenty of gatherers out there, you'll make plenty of cash if you level a gathering skill (side note again: I'm dropping mining to level LW this weekend). Random thought #5 could easily turn into its own post about making gold, but suffice it to say that you'll make plenty of money while levelling, so don't be shy spending it!
6. Get ready to raid! You can do this even as you level. By doing the daily level-appropriate 5-man, you'll be getting justice points. If you're maxed out, buy gear and stockpile it for when you're 85.
6a. Level professions. If you're on Elitist Jerks, you know that JC/LW are the 'ideal' way to go, but most professions have something to offer. Level them as quickly as you can so that by the time you're raiding, you can have one or two nice epics to go with your justice and heroic gear to help you succeed in that first 10-man. JC gives a nice trinket that's worth using, and of course LW allows you to do leg and bracer enchants.
6b. Grind reputations. If you're questing, finish off each zone. You'll get really nice reputation for questing for each faction. While most factions have something worthwhile, in particular you'll need:
-Therazane. They're your Hodir as many have said; your shoulder enchant unless you're an inscriptionist. They're easy to max. Unlock their dailies via questing in Deepholm, then do the dailies. DON'T wear their tabard; I was exalted with them three days into the expac following the above. The shoulder enchant is pretty cheap - buy several and put it on every upgrade once you hit 85.
-Wildhammer. They're your dps head enchant. It will take a little longer, but I recommend doing their dailies (in northern Twilight Highlands) and not wearing their tabard either. Why? Because...
-Ramkahen is probably your other most important rep. The epic neck you get from them at exalted will last you until heroics in Tier 11. Right now there is only one daily, so as soon as you get enough rep from questing in Uldum, get their tabard and wear it.
The other reputations aren't as important, as they can offer some decent stuff, but none of it will last that long, so feel free to level them at your leisure.
Once you're 85, do both the normal and heroic daily. If you're having trouble with heroics, do normal 5-mans until you have enough justice points for a few items - it won't take long. Heroics can - as has already been written about many times - be very challenging, with raid-like mechanics, but with familiarity, gear, and practice, they're all very doable.
It may take a little time, but you're goal should be the achievement Cataclysmically Superior (blue superior item in every slot). I got mine just yesterday, and am now officially ready for raiding!
Good luck to all in leveling and preparing for raids!