I can't speak much of 29s (or at all). But for 20s it's a little different.
Mistweaver Monks are VERY GOOD at level 20, the heals are good (Make sure to stack
https://www.wowhead.com/spell=115151/renewing-mist with
https://www.wowhead.com/spell=124682/enveloping-mist, and than spam
https://www.wowhead.com/spell=116670/vivify while channeling
https://www.wowhead.com/spell=115175/soothing-mist - All four of these healing spells have synergy), and the DPS is surprising too (Get 3 Stacks with
https://www.wowhead.com/spell=100780/tiger-palm for a massive
https://www.wowhead.com/spell=100784/blackout-kick).
Beast Master Hunter is a good level 20 spec (I prefer it in stead of Marksman).
Arcane Mage is pretty good at 20 (Not from my personal experience, I don't play mages), but they are definitely FotM.
With the CD on Priests's
https://www.wowhead.com/spell=17/power-word-shield gone, Discipline Priests are the best 20s Healers currently. Although there is a case to be made for 20s Shaman as well, but I don't know anything about 20s Shaman.