I did 80-82 in sholozar at first while GB was still buggy (cata release) then I did 81-85 (some toons didnt hit 82) once hyjal farming was more stable.. 450-525 hyjal too. I'm leveling a new DK and trying to do cobalt at 69 but sometimes get 3-4 adds (ouch) but I think should be fast.. thinking hyjal at 78 as a DK.
the 80-85 leveling mentioned was done on both herb and mining toons (4 of them, 1 warrior 1 dk 1 mage 1 shaman), the dk made it to 85 in full wrathful with very little deaths.. defiantely best char to use for farming. the mage needed the most help, warrior was pretty good once i went to fury from arms and messed with macros a bit, shaman was good once i threw in healing surge for one of the macros. DK FTW imho.
Leveling a tauren druid now too, see how that goes. If I can get back my banned shaman farmer, I'm using this new DK I'm leveling to replace my mage, dies way too much in full crafted resilience 85 gear all 85 blues.. and only in hyjal too!