Warlocks as tanks

In Mists of Pandaria, the term "warlock tank" was commonly understood to mean a Demonology warlock with Dark Apotheosis, gained with Glyph of Demon Hunting. This was removed in Patch 6.0.2.

History
Classically, the terms "warlock tank" and "drain tank" referred to an Affliction warlock with high damage to maintain aggro and talents that enhanced self-healing. Talents of note included Siphon Life (warlock talent) and Drain Life, and their combination with CC abilities like Howl of Terror allowed the warlock to concurrently survive and deal damage to a large number of mobs. However, this type of tanking was never viable for instances and raids due to the lack of protection from critical hits and spike damage from bosses that melee.

Warlocks have also served as spellcaster tanks in raids. Due to now obsolete spell resistance, an appropriately geared warlock could act as the primary aggro target of a spellcasting boss. This usually occurred in council fights where other melee targets were handled by more traditional plate-wearing tanks (Twin Emperors, Blood Prince Council). This ability is not unique to warlocks, and mages have also stepped into tanking roles. Nonetheless, the emphasis on self-healing, the existence of threat-generating abilities like Searing Pain, increased health pool, and Soul Link have always made warlocks highly capable of tanking.

In Mists of Pandaria
The Mists of Pandaria expansion introduced the Glyph of Demon Hunting, granting a Demonology warlock a new form with increased survivability. In Dark Apotheosis, the warlock gains:
 * 10% physical damage reduction
 * 15% magic damage reduction
 * a Twilight Ward that absorbs all schools of magic (instead of just and )
 * a Taunt-like ability

Like a traditional tank, Dark Apotheosis possesses many damage-mitigation qualities and the ability to keep aggro from other party members. Unlike designated tanks, warlock tanks are still missing protection from critical hits.

Though this glyph initially hinted at the viability of warlocks as "true" tanks, Lead Designer Ghostcrawler quickly quelled the community's requests to queue as tanks for LFD and LFR.

Despite Blizzard's attempts to dismiss warlock tanking as little more than a fun option with which Demonology warlocks could play around, there have been many reports of Dark Apotheosis warlocks tanking heroic instances and even raids (see video section). On the other hand, the relative difficulty of maintaining enough fury to avoid spike damage compared to using a Voidlord makes it highly situational, and pet tanking is still an important part of a warlock's PvE arsenal.

Videos

 * - Two Warlocks Tanking LFR