There are some items in the game that really do enable some unusual or otherwise completely unobtainable options or strategies. Others just give classes that don't normally have a specific ability or perk, a chance to have it or have it with a downside. This list is primarily for unlimited use items that are likely to stay with a character their whole lives.
In some ways, these items aren't game changers, but provide balance among classes, since some classes have abilities that have great utility in some situations that others lack completely.
Swimming[]
For swimming or underwater use:
[Sea Turtle]
[Hook of the Master Angler]
[Hydrocane]
[Mastercraft Kalu'ak Fishing Pole]
[Deepdive Helmet] - requires Engineering 230; recipe no longer available, but item is BoE
[Band of Icy Depths] - This item is no longer available within the game
[Azure Silk Belt] - warlocks may want to use the swim speed glyph
Falling[]
For those without levitate or slow fall type abilities:
Teleportation[]
Provides types of teleportation:
[Argent Crusader's Tabard]
[Boots of the Bay]
[Blessed Medallion of Karabor]
[Band of the Kirin Tor]
[Potion of Deepholm] - consumable but noteworthy
Multi-person mounts[]
[Reins of the Traveler's Tundra Mammoth]
[Grand Black War Mammoth]
[Grand Ice Mammoth]
[Mechano-hog] - Horde Only
[Mekgineer's Chopper] - Alliance only
[Vial of the Sands] - 2 person created from alchemy
[X-53 Touring Rocket] - flying 2 person
[Reins of the Traveler's Tundra Mammoth] - (Horde) supply & repair vendors, also 3 person
[Reins of the Traveler's Tundra Mammoth] - (Alliance) supply & repair vendors, also 3 person
Other[]
[Weather-Beaten Journal] - grants Find Fish ability
[Argent Pony Bridle] - mailbox, banking, supplies/vendors
[Jeeves] - supply & repair vendor
[MOLL-E] - portable mailbox