Just to throw in a couple of my own opinions about priest leveling.
1) This has been said before, but it's still indeed viable: Get a wand! Learn to use it, love it, and embrace it. Really makes a huge difference in my opinion.
I was late getting a wand, and when I got one at around level 20, it was all fun and games (think it was quest reward from BFD). I think I used a wand very actively until I got shadowform at 40.
2) This depends a lot on playstyle I guess, and might not be as viable at lower levels, as in higher. However I stand behind the "try to use fear, rather than shield" people. When I was leveling, up to level 66 in Outland, I always shielded up. Dots, nukes, flay etc 'till the mob's dead. It is a mana waste. I tend to only use shield in tough situations, or when fear will mess me up from adds.
I noticed this when I was farming spiders, for the silk, outside of Allerian(?) Stronghold, in Terrokar. I spent about 1,5 to 2 levels grinding them (honestly can't remember exactly, but spent A LOT of time there), making some gold - in short, being mana effective was really important to me. In the beginning using shield, dot, nukes and so on - I had to drink a lot. After a while, starting to use fear made me able to almost have zero downtime, using pet whenever it got low.
I do agree though, shield is great - and much, much, safer. For areas you *can* use fear, or feel comfortable, I strongly urge you to try it (and use it!). I then had rk 1 fear up, but that's no point anymore with 3.0.2 out though :).
In PvP, I use shield as much as I can, mana usualy as much trouble as staying alive.
Just my thoughts on this matter :)
-Vid