This tree now sports some impressive single target healing while retaining huge survivability. In solo and 5-mans, Discipline is often seen as the middle of the road between an instance healer and a grinding caster spec. If you’re leveling on a PvP server, or simply are looking for a wider range of options a Discipline build is quite a viable choice.
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Holy has always been a monster of a healing spec, but is now not a slouch anymore for solo due to the merging of stats in WotLK. Like all healing specs though, don’t expect to see a lot of Holy priests leveling anything outside of instancing.
Shadow had been, and will undoubtedly remain the top leveling spec of choice for Priests. With rock solid DPS, low downtime, and instance friendly mana replenishment Shadow will certainly be a prolific choice for leveling in WotLK.