Quality

The Quality of an item represents its rarity. All non-consumable​ items have one of five possible qualities: Common, Uncommon, Rare, Epic, and Legendary.

Bonuses

Higher-quality items have bonus attributes. The number of bonus attributes can be found by assigning each item quality to a number (with Common being 0 and Legendary being 4). This number represents the number of bonus attributes to items.

Higher-quality artifacts have bonus secondary attributes.

Higher-quality runes have higher effect scale factors. The scale factors depend on the effect.

Higher-quality weapons have bonus Standard Sockets.

In Crafting

Crafted items are of common quality by default, with a chance of crafting an item with an increased quality (not including Legendary items). Items used in crafting must be of uncommon quality or higher. Using higher-quality items when crafting gives the resulting items different bonuses, depending on the Profession.

Crafting with two rare items results in one bonus effect.

Crafting with two epic items results in two bonus effects.

Alchemy

Since Alchemists craft consumable items, crafted items do not have a quality. Instead, the chance of crafting higher-quality consumables is replaced with a chance of crafting 1 or 2 additional consumables.

Construction

When higher-quality Artifacts are used in the Construction profession, bonus secondary statistics (abbreviated to Stats) will be added to the crafted artifact.

Mysticism

When crafting with higher-quality Runes in Mysticism, modifiers (i.e. bonus effects) are added to the resulting rune.

Weaponsmithing

When using higher-quality Weapons to craft in Weaponsmithing, the resulting weapon will have more standard Chip slots.
When using higher-quality items to craft chips, the resulting chip will have bonus effects.