So I have been mainly tanking as warrior in WotLK and tried paladin for a short while, when Cata launched I leveled my warrior to 85 first, then decided to try and heal a little bit with my paladin and I enjoyed it, I only got to 82 as of now with it through the starting instances and a couple of WotLK faceroll HC's but decided I wanted to make a fresh start on leveling a healing class, so I chose Priest.
Without really thinking about it I specced Holy from 10 till 15 as I completely assumed Holy would be the prior spec for healing, however after my first Ragefire run I figured I ran OOM really fast and the spell I got for Holy spec was a DPS spell (was somewhat confused by that) and so switched to Disc for the really addicting Penance, the healing really felt a lot smoother with the mana efficient spell.
Now I'm kind of wondering what spec should I level with (both quests and instance healing), what is the most effective higher on and what are common mistakes made from people that have not been playing a healing class before?
Oh and btw, I tried the Disc spec in low lvl PvP and me and another same specced Priest completely raped the opposing graveyard in WSG, it was fun seeing enemy players thinking 'lololol a non-Spriest, let's kill it!' and then get nuked down by dots and Penance.