Faction, can't live with it, can't access Naxxramas without it. Up to now, we've really not had to worry too much about faction in the World of Warcraft. Sure, if you want to get cheap access to Naxxramas, or if you'd like to get the Ahn'qiraj rewards you'd have to get pretty far along with the Argent Dawn and Cenarion Circle, respectively. But outside of that, faction was really just a way to earn a cool mount from another race.
All of that is about to change. The Burning Crusade brings us not just a couple of expected new factions (Silvermoon for Blood Elves and Exodar for Draenei), but a whole host of interesting sounding names, such as the Ashtongue Deathswarm, Sporregar, The Consortium, Kurenai, The Violet Eye, The Mag'Har, Netherwing, The Cenarion Expedition, and a whole lot more. In fact, there are currently at least 19 new factions to gain reputation with.
Luckily, World of Raids is here to help us with a continually-updated Burning Crusade Faction Reputation Guide. From locations, to rewards, to repeatable quests, the guide is set up to have it all. If you have a chance, go check it out, and if you're in the beta, why don't you give them a hand in fleshing it out.