I figure it's not really worth the money to resubscribe again only to play for maybe a few hours a week for the next two weeks before completely locking my account until the summer. So instead, I opened up a trial account to test out my leveling theory crafting. With no gold, no party invite privileges, no glyphs, and battling server lag due to streaming, I managed to get to level 10 on Day One. Many will do better than that, since I spend half the time enjoying the scenery. Indeed, I miss Azeroth very much.
So enter Falaris, Night Elf priest on Illidan (US, PVP).
The first 3 levels are relatively easy to get through. Smite casting time is only 1.5 seconds, and it does good damage, so a mob is usually dead within 3-4 casts. Mana and health regeneration is insane, between looking at quest logs and finding the next mushroom, an empty mana bar is usually restored back to full. No drinking required.
At level 4, Shadow Word: Pain is learned. Although it costs a good chunk of mana, this spell does do good damage. At this point, my mana regeneration can still sustain casting this spell, so I went ahead and used the following cast sequence:
The trick so far (due to lack of wand), is to grab a fast weapon, rather than a high damage one. This means that if a quest offers you a mace and a staff, you take the mace unless there are green stats offered by the staff. My reasoning is this: when you attack a mob a level above you (most of the times), you will miss sometimes due to lack of hit rating. It's better to fail at a short small damage then to miss on a big hit that takes forever to execute.
At level 6, Power Word: Shield is learned. This spell is expensive, and with rank 2 Smite coming to the picture, your damage spells are no longer cheap, so you need to save mana as much as possible. Comparing with Lesser Heal, PW: Shield recovers less hp but it does prevent spell pushbacks so it's worth the mana. Rank 2 Smite surpasses SW: P in damage while costing less mana. So what I recommend from here on is to stop casting SW: P and simply go with the following rotation:
Note: If the mob runs, Smite it down before it aggros another mob. Weapon damage only when you feel it's safe to do so.
At around level 8 ~ 9, you will reach a point where if you fight mob one on one, there is no chance of dying from their damage. To save mana, stop casting PW: Shield and simply Smite (yes single button faceroll) until the mob is dead.
Some quests will require you to fight an Elite. In that case, your goal is to bring down the mob before it kills you, so bubble up, dot up, and Smite away.
I think I'm fuzzing too much over the early levels, but I think 1-20 is probably the most painful period in any priest's leveling experience, so I hope this helps.
Edit: Don't forget to buff Power Word: Fortitude.