Since its release, the most frequent complaint about Diablo 4 is "itemization." "Itemization needs to be fixed." "What the game needs is better itemization."

I've played a ton of looter ARPGs (Action RPGs) over the years (Diablo, Borderlands, Grim Dawn, etc.), but the meaning of "itemization" is kind of eluding me. Is it referring to the number of different items available? Is it referring to the level of excitement when an item drops?