1) Factions, factions factions
This is the most important part of getting started in the endgame, so the more prepared you are to transition directly into this at 80, the better off you are. There is a lot you can do to get ready before 80 without slowing down your leveling. Most factions require some type of questline to unlock, some of them quite lengthy, and all of them fonts of xp and cash.
There are 4 factions you can "champion": Wyrmrest Accord, Argent Crusade, Kirin Tor, and Knights of the Ebon Blade. Pick one of these, and get Friendly with them before you hit 80 so you can buy their tabard. You'll need to wear the tabard into level 80 instances to get rep with them. Alternatively, you can select your horde/alliance main faction (Valiance Expedition or Horde Expedition), and simply do instances without wearing a tabard to earn faction with them. All of these factions also have dailies you can unlock by doing questlines. So far, I've discovered a WA daily in Dragonblight and a major AC daily in Zul'Drak, and I'm sure there are a lot more of them around.
There are 4 other factions that you can only rep up with via questing: Sons of Hodir (frost giants), Kalu'ak (walruses), Frenzyheart Tribe (super-furbolgs), and Oracles (super-murlocs). After the initial questlines, they all offer a selection of dailies. The last two are mutually exclusive like Aldor and Scryer, except that it is very easy to switch between them. I highly recommend you unlock the Kalu'ak dailies in Howling Fjord in your early 70's, the Frenzyheart or Oracles in Sholazar Basin in your mid-70's, and especially the Sons of Hodir in Storm Peaks in your late 70's. The Sons of Hodir questline starts from a goblin in K3 with the quest "They Took Our Men!", and is one of the most fun and exciting questlines I've ever done. To give any spoiler details would be a disservice to you: go do it!
Before you hit 80, select which factions your will focus on for dailies and for dungeons based on rewards and what your expect your playtime and group availability to be. You can see the rewards for each faction through wowhead. Start here, at the Northrend faction page.
Most slots will have a decent starting Northrend enchant readily available for basic mats. You may even be able to get enchants on the cheap while they still give the enchanter skill points. But there are some slots that enchanter's can't help you with, and this is why you need to choose which factions to rep with carefully.
- Helm: As with TBC, you can buy a helm enchant at revered with the right faction. The healing, spell dps, melee dps, and tanking enchants are all spread out amongst different factions. I'll be going with the Ebon Blade, since they have the melee dps enchant. This should be your main focus when selecting your first faction to champion.
- Shoulder: Again, as with TBC, you'll get all of your shoulder enchants from the same faction. However, this time, instead of there being two varieties, one for Aldor and one for Scryer, they all come from Sons of Hodir. This is why I recommend Sons of Hodir as the first faction you focus on for dailies, at least until you get to honored. At honored, you can buy, for a nominal gold fee, shoulder enchants that match the Aldor exalted enchants. Better enchants are available at Exalted with the Sons, so you'll want to get your main character up there eventually.
- Belt: For the first time, belts have an approximation of an enchant: a belt buckle, made by blacksmiths, which adds a colorless gem slot to a belt.
- Legs: just as with TBC, you'll need to hit up a tailor or leatherworker for these. If you have one of those professions, find out which factions have the recipes you need.
In a radical change from TBC, blues from heroics are actually a tier higher than blues from regular instances. To give you an idea of how it works: a one-handed sword from vanilla Utgarde Pinnacle deals 120 dps. A similar weapon from heroic Oculus deals 130 dps. A sword from Naxxramas-10 deals 143.5 dps.
So, if you want to be maximally prepared for Naxx (which may not turn out to be necessary, based on reports of how easy it is), you'll want to gear out in heroic-tier gear (which will be a mix of blues and epics). In order to survive heroics, you will likely need to be decked out in mostly level 80 blues from the lower dungeon tier.
Meanwhile, heroic badge gear is Naxx-10 level. The same heroic badges (Emblems of Heroism) drop in heroic dungeons and Naxx-10. A different kind of badge (Emblems of Valor) drop in Naxx-25, and can be used to purchase an entirely different selection of badge items that cannot be bought with EoH's and are of Naxx-25 quality. I expect that there will be a new type of emblem for each upcoming raid tier, allowing for higher-tier badge items without the situation you had in late TBC where Tier 4 players were clearing Kara for badges to buy Tier 6 rewards.
So, start out running the level 80 dungeons, then transition to heroics to prepare for Naxx. I find it rewarding to map out my expected gear progression ahead of time, so I know which instances are best to run, and where I can save time with quest rewards, rep rewards, and BoE or craftable replacements. However, there seems to be a good mix of gear in every dungeon, so it's likely you could run them willy-nilly and still get useful drops.
Now you know how to get started at 80. If you have any questions or things you'd like me to add or clarify, I'd be glad to via the comments section below.
This counts as my Friday Laziness post, so don't expect a post tomorrow unless I'm particularly inspired. Twofer today, bitches.