Quest Druid (Untapped Potential, so the one currently in Standard – Jungle Giants already rotated out) was created in Saviors of Uldum and quickly became one of the strongest Quest decks from the expansion. It’s mainly thanks to the fact that it’s by far the easiest one to finish – all you need to do is… do nothing. Once it’s done (Turn 5-6), you can start dropping your powerful Choose One cards without having to make any choices whatsoever. After Quest is done, depending on the matchup it tries to stabilize with Taunts and healing (vs Aggro/Tempo) or put out a decent board and push the opponent (vs Control/Combo). Which is not that difficult given how powerful your Choose One cards become (e.g. Oasis Surger = 2x 5 mana 5/5 with Rush, Nourish = draw 3 cards, gain 2 mana crystals or Hidden Oasis = 6/6 Taunt that heals for 12).
The deck suffers against early game aggression – since it skips the first X turns, it’s prone to getting rushed down. Another downside is that it only has a few big threats available, so once all of them are down, games vs slow decks might be difficult. That’s why players decided to add cards such as Chef Nomi, Elise the Enlightened or Zephrys the Great to improve the deck’s late game – once you get to the bottom of your deck, you can drop those for some really powerful decks.