Item mechanics
Attack Speed
Weapons have a Slow, Fast, or Very Fast Attack Speed. Daggers, Focuses, and Wands are the fastest.
Inventory
The Inventory and Stash screens have multiple quality-of-life features, like item type tabs and smart sorting.
Item affixes
Diablo 4 has hundreds of Regular, Legendary, and Unique affixes that can roll on items of all slots.
Item durability
Item durability returns for Diablo 4, forcing players to periodically return to town and pay gold to repair items.
Item power
The Item Power value appears on Armor, Jewelry, and Weapons. It summarizes how good an item is.
Item rarity
Diablo 4 has six item rarities: Normal, Magic, Rare, Set, Legendary, and Unique. Set items won't return.
Magic find
Magic Find will return in Diablo 4, possibly as a gem bonus and also on some slot items, like Rings.
Mythic Items?
Mythic items, the formerly highest tier of Diablo 4 items, were removed by Blizzard and replaced with Uniques.
Trading
Auction house?
Diablo 4 won't include a real money or in-game money auction house. The feature was unsuccessful in Diablo 3.
Trading system
Most items can be traded, with the exception of Legendary and Unique items, and Legendary Aspects.
Currencies
Gold
Gold will have many uses, including buying items, gambling, repairing items, and trading with other players.
Murmuring Obols
Obols are used for gambling for rare items at the Purveyor of Curiosities. They're rewards for finishing events.
The game's fun depends on itemization
Many players say that itemization is the "special sauce" that makes Diablo games fun, and that Diablo 2 scored the highest marks on this. Diablo 4 brings back many of the same item qualities (e.g., Normal, Magic, Rare, Set Legendary, and Unique), but tweaks their mechanics to make all of them potentially useful. Systems like Magic Find, item durability, and class specific items also return.