What is Sidequest in Hearthstone?
Hearthstone has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Sidequest keyword.
Complete for a reward.
Sidequests are a mechanic exclusive to Descent of Dragons, and are essentially a twist on the normal Quest. Unlike normal Quests, Sidequests do not start in your mulligan and you can play multiple copies of it in your deck, and you can have more than one Sidequest active at a time. You however cannot have more than 1 copy of any given Sidequest active at a time.
As the name and mechanical nature might suggest, Sidequests are much easier to complete than regular Quests, often with very little deckbuilding required. Sidequests are only found in the "good" classes of Year of the Dragon (Druid, Hunter, Mage, and Paladin), and they each have 2 Sidequests. At least 1 Sidequest from each class summons a minion, and the second one for Druid, Mage, and Paladin has a different effect.
Cards with trigger off of normal Quests such as Questing Explorer will also trigger off of Sidequests.
Keywords that Work Great with Sidequests
There is no 1 universal mechanic that work with Sidequests. Any effect that will help you survive to complete the Sidequest will be useful.
Playing Against Sidequests in Hearthstone
As the Sidequest reward is visible to you at all times, you can save a useful card to play when your opponent completes their Sidequest (for example, saving a small AoE for Toxic Reinforcements, or removal for Strength in Numbers).
Example Hearthstone Sidequest Cards
Strength in Numbers requires the most deckbuilding out of any Sidequest, discouraging you from using small minions. However, its effect is very powerful when used in the right deck to cheat out a large minion.
Toxic Reinforcements is a Sidequest for aggressive Hunter decks (which is most of them, since it is Hunter after all). After you use your Hero Power 3 times (dealing 6 damage to the enemy hero), you will summon three Leper Gnomes (dealing 6 more damage to the enemy hero via their Deathrattles, effectively totaling 12 damage from the condition and reward).
Learn Draconic is a Sidequest for Mage that summons a 6/6 Dragon after you spend a total of 8 mana on spells.
Righteous Cause is a Paladin Sidequest which will buff your entire board upon summoning your fifth minions after playing the Sidequest. Do note that it is extremely easy to potentially miss its timing as the effect requires a somewhat large board to get the most out of the effect.