30-40

Here's my usual spots 30-40:

Finish quests in Hillsbrad (Dalaran quests/etc)
Finish in Thousand Needles (if you still have anything)
Non-Elite quests in Arathi


Start in STV around 35, do all available quests that you can solo
Run SM for equipment upgrades as needed


Very beginning quests in Swamp of Sorrows



Probably forgetting a place or two, 30-39 is an annoying stretch, only second to 40-47. After that, you're doing good in Searing Gorge/Hinterlands/Maraudon until around 51-52.