Quest World Items are generated by rarity. They range from common, to the coveted Legendary items. The more rare the item is, the better enchants and quest world attribute bonuses they provide. These items include Swords, Bows, Axes, Armor and even Shields.
Common weapons are usually weak and spawn as Wood, Gold or Iron. Common armor is Leather, Gold or Iron. These items usualy have 0-1 low level Vanilla Enchant (such as Protection 1 or Sharpness 1). Common items typically do NOT spawn with any attribute bonuses.
These items are slightly better, with 1-2 low level enchants and sometimes have small attribute bonuses.
Some rare items are good to use for newer players. These items can spawn with 1-3 decent level enchants, as well as noticable attribute bonuses. Rare items usually spawn as iron or diamond equipment.
This is where quest world items begin to get powerful. Unique gear can spawn with 2-5 medium to high level enchants, and always has noticable attribute bonuses. Uniques and above are always either diamond or iron, except for shields and bows of course. These items can be bought from the black market. Other ways to get them are rare drops from some minibosses, finding them in Boss Chests, or winning them from the Mythic Crate.
Epics live up to their name, providing numerous medium to high level enchants, as well as good attribute bonuses. These items can be bought from the black market, won from the mythic crate, seldomly won from loot crates, found inside some Boss Chests, or most commonly from killing Frozen Wastes mobs.
This tier of items is third most powerful and rare. Angelic items spawn with several high level enchants, as well as great attribute bonuses. These items are accessible in the Tier Market (in which only few people have access to), can be very rarely won in the loot crate, can be rarely dropped from certain Boss Chests, and extremely rarely dropped from Frozen Wastes mobs.
This is the second most rare tier of items. They always spawn with several high level enchants and very high attribute bonuses. Currently the only way to obtain Mythics are via the Tier Market for 1,500,000 gold per item.