Originally Posted by Auran

First and foremost, I'd say the number one way to increase your leveling speed assuming you're soloing and not getting power-leveled in any way is to increase your movement speed. In that light, I can make a few suggestions with various assumptions built in (assume you can afford respec costs a couple of times while leveling, etc):

Druid: Getting the Tier 3 feral talent to increase your cat movement speed. You can max this out like 9 levels before you get travel form. Avoid the Shredding Attacks talent, you won't be shredding much as a soloer.

Hunter: Similarly to Druid, Tier 3 BM talent to increase your pack speed. Not huge, but can save you time. On a similar note, I've found a pet with Screech to be the end all, be all of leveling/grinding. It helps for mutliple targets and is great aggro generation/focus dump.

Priest: Until you get Shadowform, you'll be using your wand...a lot. Believe it or not, Wand Spec is powerful and important. At the early level (10-15) it doesn't matter too much which you go for first, Wand Spec or Spirit Tap, but once you hit 40, you'll want to respec Shadowform and then your next 5 levels, I'd go back and grab Wand Spec again.