So far, we have reputation rewards for Therazane the Stonemother in Deepholm, the Baradin Watchers and Hellscream's Reach in Tol Barad, the Wildhammers and Dragonmaw Clan in the Twilight Highlands, and the Ramakhen faction of Tol'vir from Uldum. Right now, all rewards are itemized for Alliance in the case of the Hellscream's Watch and Dragonmaw, but new models and names should come in a later build. While these are not necessarily the complete lists for each faction, you get an idea of what will be available.
Each faction continues the trends established in Wrath. Therazane is the Sons of Hodir-style faction, with a host of shoulder enchants available. It's interesting to note that while there's only one spell shoulder enchant available, there are two for physical DPS, one a strength DPS shoulder enchant for the plate DPS classes and one with agility for hunters, rogues, feral druids and enhancement shamans. It's also interesting that the agility enchant has mastery, while the strength one has critical strike rating. There are also a variety of rings available. One thing that I found interesting is that the tank ring has no more stamina than the DPS ring. If I were a feral druid, I'd definitely consider that DPS ring for tanking, in fact. Agility, hit and mastery all seem pretty solid.
The Wildhammer/Dragonmaw offer a strength physical DPS enchant for the helm, a solid-looking spell DPS neckpiece, and armor pieces from cloth to plate for almost any role. The Ramakhen offer the agility DPS helm enchant and various gear as well, but frankly, it's the camels I expect to be the big draw here. Get 'em in grey, tan and brown.
Tol Barad, meanwhile, looks to be the place you're going to go to get your starting weapons at 85. Whether you're looking for a 2H sword, a polearm, a staff, a melee dagger, a 1H axe, a crossbow, or a healing mace, they have you covered. However, in addition to all of that, there are five epic trinkets (strength melee, agility melee/physical, tank, spell DPS, healing), two epic mounts, and a noncombat pet to drive lots of interest and attention this way. While Therazane will be the faction you most likely have to grind, Tol Barad has the goodies to make you want to grind it.
It's possible there will be more factions and/or more items for each, so don't assume these lists are final in any way.