Torment-only

Torment-only refers to game features and items that are only available on the Torment 1-6 difficulty levels. Most things in Diablo 3 and Reaper of Souls scale up with difficulty and thus grow more rewarding on Torment, but "Torment-only" refers to items and systems that do not exist at all on lower difficulties.

Torment Required

Torment 1 and higher enables the following:

Diablo 3 Class Item Sets can only drop on Torment 1 and higher, at level 60 and higher.
New level 70 legendary and set items in Reaper of Souls only drop on Torment 1 and higher.
Legendary item drop rates get a special boost in Nephalem Rifts on Torment 1 and higher.
The Infernal Machine only exists on Torment 1 and higher. The keys and organs do not drop below Torment, and the Hermit's Shack can not be entered below Torment.
Imperial gems may drop instead of Marquise gems on Master and higher.

Torment Only Legendaries

The full list of Reaper of Souls Torment-only items was revealed in February 2014.[1] This list was added to a few weeks later when all of the Diablo 3 class-specific sets were also tagged as Torment-only. That applied to their drops in D3v2 or RoS, and made them the only Torment-only items in Diablo 3.

Diablo 3 Class-Restricted Item Sets

Immortal King -- Barbarian set.
Natalya's Wrath -- Demon Hunter set
Inna's Majesty -- Monk set
Tal Rasha's Wrappings -- Wizard set
Zunimassa's Spirit -- Witch Doctor set

Reaper of Souls Class Restricted Item Sets

The Legacy of Raekor (Barbarian Set)
Firebird’s Finery (Wizard Set)
Embodiment of the Marauder (Demon Hunter Set)
Raiment of a Thousand Storms (Monk Set)
Armor of Akkhan (Crusader Set)
Helltooth Harness (Witch Doctor Set)

Theme Sets (Non class-restrictive)

Legendary Follower Items

Smoking Thurible (Enchantress)
Hand of the Prophet (Enchantress)
Skeleton Key (Scoundrel)
Ribald Etchings(Scoundrel)
Enchanting Favor (Templar)
Relic of Akarat (Templar)

Legendary Potions