Reputations work a bit differently in Wrath, mainly due to the idea of championing. If you're looking for a detailed explanation, check out Alex's article on the topic, he gets into the nitty gritty of how it works. I'll be covering the specific pieces of gear that are of interest to Rogues: weapons and armor. The first two factions we'll explore are the Horde Expedition and Alliance Vanguard. Blizzard has thankfully revived the blood feud between the two main factions of Azeroth, even splitting Dalaran into sections for each. I hated the Scryer vs Aldor motif of TBC; it just didn't feel epic or like I was really a part of something greater than myself.
Horde Expedition & Alliance Vanguard: OH Mace, Quick Dagger.
The Horde Expedition has 2 items of potential use for Rogues, and both are level 78 blue weapons. The Warsong Punisher and Warsong Shanker are both very strong weapons, although the Punisher looks to have been designed more for a tanking class. It's best suited for an off-hand to a Mace Rogue, of which there aren't too many after the changes to Mace Specialization. The Shanker, on the other hand, screams for us to do naughty things using it. With a quick 1.5 speed and all the right stats in all the right places, it compliments any spec as an off-hand, or can be used for a PvE Mutilate main-hand if you don't have anything quicker. The Alliance Vanguard's rewards mirror these, they're simply renamed: the Hammer of the Alliance Vanguard and the Vanguard Soldier's Dagger.
There are also several neutral factions that we can align ourselves with in Northrend. There's only one "faction choice" similar to the Scryer/Aldor factions in Shattrath: The Frenzyheart Tribe and their enemies, the Oracles.
Frenzyheart Tribe: Pants, Trinket, Throwing Weapon
The Frenzyheart Tribe is a group of Wolvar (think werewolf) that will allow you to join their fold through a series of quests. As far as reputation rewards go, these guys have their counterpart, the Oracles beat pretty badly, at least for Rogues. Not only do they have some awesome Azure Strappy Pants and a Frenzyheart Insignia of Fury, they also host the Stolen Vrykul Harpoon throwing weapon. I will be allying with the Frenzyheart's for this thrown weapon alone. Finally, a throwing weapon that doesn't look like we bought it off a level 1 vendor! Plus, they're werewolves. I think wolves and Rogues have a lot in common, but if that doesn't convince you, take a look at the Oracles.
The Oracles: Belt, Trinket.
First discovered by Mohinder Suresh, The Oracles are considered to be an evolutionary step-up from Murlocs; I imagine some people will align with them just to avoid being zerged by the sound of mrrrglglglrmgl while walking around Sholazar basin. Their trinket offering, the Oracle Talisman of Ablution is arguably the better trinket when compared to the Frenzyheart's. They've also got a fairly good belt option in the Glitterscale Wrap. But they have a wand instead of a throwing weapon, which is an instant turn-off.
Kalu'ak: Slow Dagger, Chest, Fishing Pole
Our next faction is the Kalu'ak tribe. They're a race of tuskarr (walruses) that are really good at hunting and fishing, and not so good at being sneaky. They've still got some great Rogue rewards though, starting with a lower-level leather chest made from whale skin. The Traditional Flensing Knife has one of my favorite weapon models (it looks like a fish hook), and is a good slow Mutilate dagger for PvP builds. Their final reward is a Mastercraft Kalu'ak Fishing Pole, which is not only awesome for catching fish; it also allows the user to breath underwater. Rogues had been excluded from the Hydrocane club due to its 2-hand requirement, and so now we will have our revenge!
We'll now be moving to the more "end game" factions that offer level 80 rewards as well as epic items.
Argent Crusade: Cloak, Boots
The first of these factions is the Argent Crusade, who were formed by the merging of the Knights of the Silver Hand and the Argent Dawn. They're leading the fight against Arthas, and offer some of the best gear to aid in that conflict. They have a decent Cloak of Holy Extermination available, but their strongest offering are a pair of Boots of the Neverending Path. These are stacked with enough hit rating for 2% hit at level 80 and some awesome AP and agility to boot (zing!).
Kirin Tor: Quick Dagger, Pants
The Kirin Tor are unfortunately a caster faction, so don't expect any epic rewards from them. Their blue offerings are very, very sweet to make up for this inequality. Their Lightblade Rivener is an insanely good Mutilate dagger that's non-unique, so pick up two and get raiding – it only requires honored reputation! They've also got a pair of Mind-Expanding Leggings that offer enough Expertise Rating to reduce your chance to be dodged/parried by nearly 2%. That's a 4% increase in frontal DPS (2% parry AND 2% dodge reduction), which is great when soloing. It also helps avoid getting "parry gibbed" when a boss parries you, and helps against enemy players who have higher dodge/parry than your typical mob.
Wyrmrest Accord: Epic Bracers, Quick Sword
The Wyrmrest Accord seems to be the faction of choice for tanks, with mostly plate and tanking rewards. They do offer the epic Dragonfriend Bracers which are fairly standard issue as far as stats go; and the Fang of Truth, which every Combat Sword Rogue will be after as soon as possible. It's top-tier as far as sword off-hands go in Northrend.
Knights of the Ebon Hold: Epic Chest, Slow Sword, Helm Enchant
After Arthas takes leave of Ebon Hold, the Knights of the Ebon Blade take over. They're the faction behind Death Knights, and if there's one thing that DKs know how to do, it's hit stuff hard. With the epic Darkheart Chestguard and its 4.5% armor penetration coupled with the Reaper of Dark Souls completing the main-hand Sword for your Combat set, you'll want to work on leveling your faction with them early. They also house our Arcanum of Torment, the helm enchant of choice for any Rogue.
Sons of Hodir: Slow Dagger, Shoulders, Shoulder Enchant
The Sons of Hodir complete our study of the factions, while also bringing the highest quality rewards. The reputation grind with them is similar to the Netherwing faction of TBC, but the rewards are definitely worth the effort. The best reputation PvP dagger is their Broken Stalactite, and I'm sure we'll be seeing a lot of Assassination and Subtlety Rogues wielding these in the weeks to come. The Sons of Hodir not only offer the Greater Inscription of the Axe, our shoulder enchant; they even have a pair of decent blue shoulders for us to put the enchant on!
As you can see, there are a ton of great items available for us Rogues in Northrend. A pair of weapons for any Sword or Dagger spec, and every armor slot save Helm, Gloves, and Rings. When on your way to 80, focus on the pieces that you are weakest in, and try working towards some of these more-than-ample rewards. You don't have to swear your fealty to Tirion or Rhonin, but you will have to help them "take care" of some of their problems in the way that only a Rogue can. So sharpen your blades, and get ready to do what you do best.