Lane points out something interesting that I hadn't yet noticed after 2.0: faction reputation via quest is not subject to level any more.
It's true. I just loaded up my 55 night elf hunter, ran a quest in Northshire Abbey, and sure enough, 20 seconds of killing kobolds got me 250 rep for Stormwind. Most players trying to grind reputation for another racial faction (usually for the mounts, but most vendors will also give a small discount for higher rep) were limited to grinding around for tons and tons of runecloth, but it appears that doing lowbie quests will now grant just as much, if not way more, reputation. You won't be able to see the "!" or earn XP for them (if you're doing quests way below your level, that is), but the rep is there and plentiful-- just talk to the questgiver.
I've never heard of this, so there's a chance that Blizzard hasn't as well, and that this is just a bug waiting to be fixed. But for now, if you've been waiting to grind up reputation for a mount, just head back to the starting area of your choice, and have at it.