I've been leveling a few alts by rest-rotating. Warlock just hit 80. I tried switching my priest to shadow but it was just like the affliction lock only not as fun and no pet. Only classes I haven't leveled are Shaman and Warrior, and I'd say Ret Paladin/Blood DK/Affliction lock are the fastest so far due to absolutely no downtime when played correctly, plus the ability to handle elites.