As a newly dinged level 80 hunter I found factions to be daunting and overwhelming.
Tabards, quests, instances, HALP!
After doing some research I've tried to compile a list of the better factions and which ones I recommend you head for first.
1) Argent Crusade
The plague killing good-doers were a **tch to get rep with pre - BC. However, if you do a good deal of questing in Icecrown you should be close to or honored with Argent Crusade by the time you hit 80. Once you hit honored you can pick up
Cloak of Holy Extermination which is one of the better entry level 80 back pieces available. After some instance or quest grinding you can hit revered which offers Zombie Sweeper Shotgun
which is one of the nicer ranged weapons if you havn't coughed up the mats or cash for a nessingwary 4000.
Exalted is where the fun begins. Boots of the Neverending Path and Polished Regimental Hauberk are some of the better pre-25 raid drops you can get.
2) Knigts of the Ebon Blade
Questing in Icecrown also gives a nice boost to starting rep for Ebon Blade. There are also three daily quests available right off the bat that are easy to complete, fast, and boost your rep.
Arcanum of Torment is a great head piece enchant, granting +50 AP and 20 crit rating, and Design: Wicked Monarch Topaz for you jewlcrafters out there, these can both be bought with revered status, as well as Spaulders of the Black Arrow. Once you hit exalted, Darkheart Chestguard becomes available which is a very sturdy chest piece, even though its leather.
3) The Sons of Hodir
Difficulty: Semi-Pain inducing
The quest chain to become neutral with the Sons of Hodir is very, very long. The quests however are extremely easy and go by quickly. See the comment made by Mumpley for a very detailed explanation on how to get neutral status. The Sons of Hodir offer some very nice equipment. Once honored, you can get your hands on Lesser Inscription of the Axe, and Giant Ring Belt. Revered offers the much wanted Ice Mamoth and Stalactite Chopper if you're a dual wielder. Exalted offers an updated axe inscription, Greater Inscription of the Axe, Reins of the Grand Ice Mammoth, and Design: Smooth Autumn's Glow.
These three factions are the ones you should focus on as a fresh 80. I'll try to cover some of the other factions and rewards some other time.