There are a lot of new factions in the Burning Crusade. Just by looking at WoWWiki's factions page, I can count thirteen, and that's combining all the opposed faction ones like Honor Hold/Thrallmar, Mag'Har/Kurenai, and Aldors/Scryers.
With so many factions, it's nearly impossible to grind rep with all of them. So I've come up with a game plan. I have to get revered with Thrallmar, Lower City, the Sha'Tar, Cenarion Expedition and Keepers of Time to get my Mount Hyjal attunement, so those are a given. That alone should net me a couple of transmutes and alchemy recipes.
Aside from that, I have two factions I am considering grinding to exalted: the Scryers and the Consortium. They both have excellent daggers available at exalted, and I'd like to be able to start the endgame off with some good daggers. The Keepers of Time also have a nice dagger, but they seem significantly harder to grind solo than the other two. Plus, the Consortium is a group of ethereal smugglers! How cool is that?
After I've been 70 for a while, I may go back and do some exalted grinds to pick up alchemy recipes, but I doubt I'll be in any hurry. What factions do you want to grind out, and what reputation rewards are you lusting after?