I leveled mine as Resto but for questing I ran with Balance. With the Moonbeast Glyph (cast heals in Moonkin), puting DoTs on all mobs pulled. Starsurge Proc's at a good rate and kiting is very easy!! it breaks away from just standing still and mashing buttons!
I leveled feral from 85-90 and had no issues. I don't know how leveling as guardian is but I've seen a few bears around while leveling. It's probably great for doing dungeons since queues for dps can be long.