Unique items, the god drops of Diablo 4
Blizzard has said that Unique Items are so powerful, players will base entire builds around them.
Number of affixes
Unique items have 5 affixes, with one of those being a Unique affix, which is multiple times more powerful than a regular affix.
Drop rate
Uniques are so rare that players can expect only a few to drop each Season. Regular and Sacred Unique items begin dropping in World Tier 3. Ancestral Unique items only drop in World Tier 4.
Sacred and Ancestral versions
Both Sacred Unique items and Ancestral Unique items exist, with the latter being the most powerful (Item Power ~800). They'll have the same affixes as regular Uniques, but higher stats.
Trade status
Unique items are account-bound, so players can't trade them.
Number of Unique items
Diablo 4 is expected to launch with between 50-100 Unique items, many of them Class-based. More will be added in Seasons and expansions.