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.
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.
Unique items are account-bound, so players can't trade them.